Genoise Sponge Cake Recipe

Low-Carb Genoise Sponge Cake:

4.5 ounces of flour substitute (I use 3 parts almond flour and 1 part coconut flour, plus 1 teaspoon of guar gum per each 1 cup of flour substitute)
2 ounces butter
1/2 teaspoon salt
6 eggs
7 ounces sweetener (I recommend a mix of mostly erythritol or xylitol, with a little bit of maltitol or isomalt for structural support)
1 teaspoon vanilla

This recipe makes a 18" by 13" sheet cake or one 9" round cake.

"Set a saucepan big enough to hold your mixer bowl on the stove with an inch of water in it. Bring it to a simmer. Meanwhile, sift the flour and salt together into a medium bowl. Melt the butter in a small saucepan and set aside.
Pour the [sweetener] into the mixer bowl, add the eggs and whisk to combine. Set the mixer bowl over the pan of water and heat the mixture until it’s warm to the touch (no more than 120 degrees). What you’re trying to do is simply melt the sweetener…don’t cook the eggs! It’ll only take a minute or so.
Remove the bowl from the heat and, using the paddle attachment, beat the mixture on medium high until it’s very light and foamy, about triple its original volume (this will take up to ten minutes with a stand mixer). Add the vanilla and beat an additional 10-15 seconds.
Pour a cup or so of the egg foam into the cooled melted butter and stir it until it’s completely incorporated, then gently pour the mixture back into the mixer bowl (this eases the incorporation of the butter into the batter). Next, sprinkle the flour substitute mixture into the mixer bowl and carefully fold (instructions under the “Techniques” menu to the right) until the flour mixture and the butter mixture are completely incorporated. Gently pour the batter into the prepared pan.
Bake until the cake is a pale gold color and springs back lightly when touched." -directions from JoePastry site linked above