Keto marble cake doesn't get any better than this! Two layers of low carb vanilla and chocolate cake swirled together, with a silky smooth low carb chocolate frosting. A perfect keto birthday cake recipe.
Preheat the oven to 325F and grease 2 8-inch round baking pans. Line the bottoms of the pans with parchment and grease th parchment.
In a medium bowl, whisk together the almond flour, whey protein, baking powder, and salt. Set aside.
In a large bowl, beat the sour cream with the sweetener until well combined. Beat in the eggs and vanilla extract. Add half of the almond flour mixture and beat until combined, then beat in the almond milk. Beat in the remaining almond flour until the batter is well combined. If it's very thick, add another tablespoon or two almond milk. It should be spreadable but not pourable.
In another large bowl, whisk the cocoa powder and hot water together until smooth. Add about 1/3 of the vanilla batter to the cocoa mixture and stir until well combined.
Add the two batters in small alternating dollops to the two pans and swirl with a knife, then smooth the tops. Bake 25 to 30 minutes, until the edges are set and the tops are just firm to the touch.
Remove and let cool completely in the pans, then flip out onto a wire rack and remove the parchment.
Set a heatproof bowl over a pan of barely simmering water. Add the chopped chocolate and stir until melted and smooth. Remove from over the pan and let cool while preparing the rest of the frosting.
Prepare the Sugar-Free Swiss Buttercream according to the directions. After all the butter has been added, set the mixer on low and drizzle in the melted chocolate until combined. You will need to stop the mixer and scrape the sides and bottom with a rubber spatula a few times to get all the chocolate mixed in.
Note: if your frosting is super soft after adding the chocolate, simply place in the fridge for about 5 to 10 minutes to firm up a little before frosting the cake.
Set one layer of cake on a serving platter and spread the top with about 1/3 of the frosting. Place the second layer on top and spread the top and sides with more frosting. There should be enough to pipe some rosettes on the top as well, if desired.
Decorate with mini keto cookies or small pieces of sugar-free chocolate. Or consider adding some sugar-free sprinkles!