Preheat the oven to 350°F (177°C). Line the bottoms of two 9-inch layer cake pans or one rectangular 9- X 13-inch pan with parchment paper.
Melt the chocolate over very low heat until it's almost melted, stirring occasionally. Remove it from the heat and stir it until it's completely smooth.
Whisk together the flours, baking powder, baking soda, salt, and psyllium husk together in a medium mixing bowl.
With a mixer, beat the butter in a large mixing bowl until it's smooth and creamy. Gradually add the sugar and continue beating until it's light and fluffy. One at a time, beat in the egg yolks, vanilla, and melted chocolate, scraping down the sides as necessary. Add the flour mixture in three parts, alternately with the milk in two parts, and stir just until smooth.
With clean beater in a clean bowl, beat the egg whites until soft peaks form when you lift out the beaters. Gently but thoroughly, fold them into the batter. Spread out the batter into prepared pans.
Bake in preheated oven for about 20 to 25 minutes or until internal temperature reaches 200°F (93°C). Set on a wire rack to cool for 10 minutes before removing from pan
Melt chocolate chips over very low heat until just melted, stirring occasionally. Stir in the nut butter, and frost the cooled cake.
This recipe makes a 9- X 13-inch (23 cm X 33 cm) sheet cake or two 9-inch (23 cm) layers.
Use either xanthan gum or psyllium husk powder to replace gluten in holding the cake together. Psyllium husk powder will give the cake a little more crumb.
To "fold" in egg whites, stir a large scoop of them into the batter. Add the remaining beaten whites and proceed to stir them in by gently drawing a spoon through the batter down one side, across the bottom, and up the opposite side. Turn the bowl a little at a time and repeat this motion until the egg whites have been completely, but gently, incorporated.
Be careful not to overbake your cake, especially if you use psyllium husk powder.
Store cake that is not eaten the first day in the refrigerator or freezer.