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)
- 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
- 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 , 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.