Keto lemon layer cake with lemon curd and mascarpone frosting
- 6 6 large egg yolklarge egg yolks
- ½ cup (2¼ oz.) 120 ml (65 g) powdered erythritol
- ½ cup 120 ml lemon juice
- 1 tbsp 1 tbsp lemon zest, finely grated
- 3 oz. (6 tbsp) 85 g (90 ml) unsalted butter, cubed
- 2⁄3 cup (2¼ oz.) 160 ml (65 g) coconut flour
- ½ cup (1¾ oz.) 120 ml (55 g) oat fiber
- ¼ cup (¾ oz.) 60 ml (25 g) whey protein isolate (unflavored)
- 2 tsp 2 tsp baking powder
- ½ tsp ½ tsp salt
- 8 oz. (1 cup) 230 g (240 ml) unsalted butter, softened
- 2⁄3 cup (5 oz.) 160 ml (140 g) granulated erythritol
- 8 8 large egglarge eggs
- 1 cup 240 ml heavy whipping cream
- 2 tsp 2 tsp vanilla extract
- 2 tbsp 2 tbsp lemon juice
- 2 tbsp 2 tbsp lemon zest
- 5-6 drops liquid sweetener, to taste
- 2 cups 475 ml heavy whipping cream, chilled
- 1 cup (4½ oz.) 240 ml (130 g) powdered erythritol
- 2 tsp 2 tsp vanilla extract
- 1 lb (2 cups) 450 g (450 ml) mascarpone cheese, chilled
- 1 1 large lemon, finely grated zestlarge lemons, finely grated zest
- In a saucepan over low heat, whisk together the egg yolks, sweetener, lemon juice, and lemon zest. Whisk constantly until the mixture reaches 170°F (75°C) on a candy thermometer and begins to thicken. If you don’t have a candy thermometer, you will know that the curd is thick enough when it coats the back of a spoon and begins to cling to the whisk. Remove from the heat.
- Add the butter, one cube at a time, whisking constantly to melt the butter and incorporate it into the lemon curd.
- When the butter is fully incorporated, strain the curd to remove any lumps or larger pieces of zest. Cover with plastic wrap, pushing the wrap gently onto the top of the curd to prevent skin from forming.
- Let fully cool for at least an hour before assembling the cake.
Lemon layer cake
- Preheat the oven to 350°F (175°C). Liberally grease two 8-inch round cake pans and line the pans with parchment paper.
- In a large bowl, whisk together the coconut flour, oat fiber, protein isolate, baking powder, and salt; set aside.
- In a separate large bowl, use a hand mixer or stand mixer to whip the butter and granulated sweetener. Add the whole eggs one at a time, beating well after each addition. Stir in the heavy cream, vanilla extract, lemon juice, lemon zest, and liquid sweetener by hand. Add the dry ingredients and mix by hand until just incorporated.
- Divide the batter evenly between the prepared cake pans. Bake for 20 to 25 minutes, until a toothpick inserted in the middle of a cake, comes out clean.
- Let the layers rest for about 10 minutes before removing from the pans to cooling racks to cool completely.
Whipped mascarpone frosting
- To make the frosting, start by whipping the cream with a hand mixer in a large mixing bowl.
- As you whip the cream, sprinkle in the sweetener and extract. When thickened, add the mascarpone and whip just until firm.
- Stir in the lemon zest by hand and refrigerate until ready to frost the cake. If you refrigerate for longer than 30 minutes, the frosting may become too firm to work with easily.
Assembling the cake
- To assemble, slice each cake round in half horizontally.
- Place one layer on a cake plate or cardboard round and spread 1/3 of the lemon curd on top. Use a light hand or a piping bag to cover the lemon curd with a thick layer of mascarpone frosting.
- Add another layer of cake followed by another 1/3 of the lemon curd and another thick layer of frosting. Repeat until the four layers of cake are assembled.
- Cover the top and sides of the assembled cake with the remaining frosting.
- Garnish with additional lemon zest if desired.
- Cover and refrigerate for at least 8 hours or overnight before serving.
When zesting the lemons, be careful to include only the yellow zest; the white pith underneath is bitter-tasting and will affect the flavor of your curd.
You can make powdered sweetener by putting granulated sweetener in a clean, dry blender or coffee grinder. Measure after powdering the sweetener.
Storing the cake
If covered well, the cake will keep in the refrigerator for up to 5 days.
Yes, you may wish to slice it and then freeze the individual portions.
(1) under Kristie's tips, it says to reserve the egg whites for the cake layers, but the recipe calls for 8 whole eggs. Is that a typo?
(2) should the bake time be adjusted if using 4 of the 8" layer pans?
I made the amount of the frosting for 5 servings of cake and split that in two for me and hubby.
So I rated it 5 stars but have only tried and tested the frosting... No fault with that!! 😁
Oat fiber is the insoluble outer husk, not part of the oat itself. You may be able to get it from Amazon or iHerb.com
Unfortunately there is not a good substitution for oat fiber.
I found it difficult to get the eggs and butter incorporated properly. Took a lot of mixing and I ended up switching to the whip attachment for my kitchen aid to get a good consistency before adding the dry ingredients. After baking, the pans were quite buttery on the bottoms and the cakes did not rise well. Most likely due to lack of mixing the dry ingredients well enough. When I try this cake again, I will do a better job of whisking the dry ingredients and make sure that my butter is just a bit softer before starting. The cakes were difficult to slice since mine hadn't risen well but i made it work. The crumb of the cake is course, similar to a sponge type cake, but delicious just the same. I'm sure it will be better next time with the changes I intend to make! I've made lemon curd plenty of times before, never had one turn our quite this dark yellow but it tasted delicious and you'd never know that it is sugar free. The "frosting" was easy to make although where I live, expensive. I'm serving the cake for Easter Dinner tomorrow and looking forward to my family's comments. I think they will be pleasantly surprised that mom's weird keto dessert will taste so good!
The left over egg whites were used to make keto bread.
I could not find Oat Fiber in time to make this, so I substituted oat bran, which didn't seem to hurt anything. The cake does not rise as well as a "normal" cake, but it was still fine. The lemon curd: I had problems with this - it basically oozed out of the layers as it didn't thicken enough, in spite of my best efforts. Had I known it wouldn't hold up on the cake, I would have added Xanthum gum to thicken before putting on the cake. The frosting is to die for, so good. However, next morning, belly not too happy. Probably too much erythritol, overall, for my digestive track (between cake and curd and frosting). I will probably have to throw out what has not been consumed, as I had this problem two days in a row, even with 1/20th serving of the cake (very thin slice).
Unfortunately, that may have been the oat bran. Oat bran is not a good keto substitute for oat fiber. Oat fiber is the completely insoluble outer husk, not part of the oat itself.
I am not sure, we have not tested this in a larger pan.
We have not tested this with cream cheese and mascarpone. They have different consistency and taste.
Not only is it still not consistent if they cause you to jump out of ketosis or not (it seems to have done that on one test Endfeldt did, but that's a anecdote). They are also fairly expensive.
Avoiding desserts is one easy way to accomplish this! Our recipe filters can select main or side dishes and you can avoid desserts.
We have not tested this with cream cheese instead of the mascarpone. Since it's just in the frosting and doesn't need to be baked, it may work though may result in a much "heavier" frosting.
You can leave out the oat fiber and the sweetener when calculating macros as they don't affect blood sugar.
This should work well for piping!
Adding additional granulated sweetener also adds to the volume of dry ingredients. You can taste the batter to see if you think it's sweet enough before adding liquid sweetener if you prefer.
Instead of the oat fiber (not sold in the stores we buy our stuff), we used flaxseed meal (the mentioned amount for oat fiber) and guar gum (1%of the total dry ingredients weight for the cake layers). The latter ingredients we use more regularly to prepare our non-wheat-containing dough.
We did not use any 'liquid sweetener' and the specified amount of erythritol made it a bit too sweet, but no worries, still great taste amazingly filling and something else for a difference.
Thank you for letting us know how much you enjoy this recipe!
I would be very grateful for an answer, because I would like to bake this cake for a birthday in next week.
Hi, Malou! You should be able to add more coconut flour instead of the oat fiber.
Hi, JT! I checked with our recipe team, and there haven't been any recent changes to this recipe. Perhaps you are thinking of a different recipe?
While we have not tested this substitute, sometimes egg white protein powder can be used in place of whey protein isolate in recipes, but sometimes it can make the baked good texture more rubbery. The protein in whey protein isolate helps replicate what the protein in gluten in wheat flour does - it helps to give lift, rise and structure to baked goods to help prevent them from being so dense.