Preheat oven to 350°F. Spray 3 8-inch round cake pans with non stick spray and line the bottoms with parchment paper.
In a small bowl, mix together the cocoa powder and hot coffee (or water). Stir until the cocoa powder is dissolved and then set aside.
In a large bowl, whisk together the gluten free flour, cornstarch, xanthan gum, baking powder, baking soda, salt, and granulated sugar then set aside.
Add the egg whites to a medium bowl then add the cream of tartar. Beat with an electric mixer on medium-low speed until the egg whites become thick and foamy then set the bowl/egg whites aside.
In a separate medium bowl, whisk together the egg yolks, buttermilk, melted butter, oil, and vanilla extract. Pour the wet ingredients into the dry ingredients and stir until incorporated.
Fold half of the whipped egg whites into the batter. When there are only a few white streaks left, add the remaining egg whites.
Gently fold in until no white streaks remain. Make sure you're folding and not stirring/mixing so you don't deflate the egg whites. It may take a minute or two to fully incorporate.
Transfer the batter to the prepared pans, making sure it's distributed evenly between the three and spread the batter evenly in the pans.
Give the cocoa mixture a stir and then drizzle over the cake batter, dividing it between the three pans as evenly as possible. Then use a knife of wooden skewer to swirl the chocolate into the yellow cake batter.
Bake at 350°F for 20-22 minutes or until done in the center. Don’t over bake!
Cool the cakes in the pan for 25 minutes then remove them from the pans and cool completely on wire racks. Once the cakes are cooled, frost them with chocolate buttercream frosting or frosting of your choice.
I use my Nightshade Free flour blend in this recipe. If you use a flour blend that contains xanthan gum, omit the amount called for in the recipe.