Preheat the oven to 350 degrees F. Prepare two standard 8-inch cake pans by lightly spraying the bottoms and sides with cooking spray. Cut out circles of parchment paper to fit on the bottom of each pan. The paper should be flat, not inching up the sides of the pan. Set pans aside.
Mix together the cake flour, cocoa, baking soda and salt in a bowl with a whisk and set aside.
Combine the chocolate, brown sugar and buttermilk in a stainless steel bowl and place over simmering water. Stir the ingredients together as they melt. Transfer to a large mixing bowl.
Put the butter in the bowl of an electric mixer fitted with the paddle attachment (or use a mixing bowl and hand-held mixer), and mix at medium speed until the butter is light and soft. Change to the whisk attachment (if using bowl mixer) and slowly add the granulated sugar and continue whipping until light and fluffy. Add the egg yolks, one at a time, and beat until each is incorporated.
Add half of the dry ingredients and mix in slowly. Add half of the buttermilk. Repeat this process until all the flour and buttermilk mixtures are incorporated. Add the batter to the chocolate base and combine lightly using a large rubber spatula. Stir in the vanilla, then set aside.
Thoroughly rinse and dry mixer bowl and whisk attachment, then mix the egg whites at medium speed for about 3 minutes, or until they hold a medium peak. Gently fold half of the egg whites into the chocolate batter until the mixture is smooth. Repeat with the remaining egg whites, and fold just until all the egg whites are incorporated. There should be no streaks.
Pour the batter into the cake pans. Bake for about 35 minutes or until the cakes spring lightly in the center when touched, or a toothpick comes out clean with a few crumbs but no batter.
Remove the cakes from the oven and cool on a rack until room temperature. Remove the cake layers by using a paring knife to gently cut the cake from the sides of the pan. Take a cake circle or sturdy paper plate and place it over the pan. Invert the cake onto the plate and remove the parchment circle. Repeat this process with the other cake layer. Ice the first layer with the icing of your choice, with about 1/2 inch of icing on the top. Place the second layer onto the cake with the bottom side up, which gives you a flat, undomed top. Ice the whole cake thinly. Chill cake in the refrigerator for a t least 20 minutes, then apply a final coat of icing.