6 large eggs, separated
1/2 cup unsalted butter, softened
1 cup brown sugar
2 tsp vanilla
1/4 cup milk
1 cup coconut flour
1/2 cup tapioca flour
1 tsp baking soda
1/2 tsp baking powder
1/4 tsp salt
2 Tbsp cocoa powder
1 Tbsp water
Preheat the oven to 350 degrees F. Grease and flour a 9- to 10-inch bundt pan.
In a mixing bowl, beat egg whites until fluffy.
In another bowl, cream the butter and sugar; add the egg yolks, vanilla and milk until well combined. Add the coconut flour, tapioca flour, baking soda, soda powder and salt. Mix again. Fold in the egg whites.
Pour two thirds of the batter into the prepared pan.
Add the cocoa powder and water to the remaining third of the batter and whisk to combine. Pour the batter in the pan and gently swirl the batter with a butter knife. You don't want to mix the batters together, you just want to create a marbling effect.
Bake until the cake is set and slightly browned, 30 to 35 minutes. Cool before unmolding.