CAKE

1. Preheat oven to 350 degrees.  Line two 9-inch cake pans with wax or parchment paper.  Butter the paper and insides of the pans.  Lightly flour the pans, tapping out excess flour.  Set aside.
2. In a medium-size bowl, whisk together the cake flour, salt, baking soda, and baking powder.  Once whisked together, sift the dry ingredients into a separate bowl, to remove and small lumps in the cake flour.
3. Add the butter and sugar to the bowl of a stand mixer fitted with the paddle attachment.  Cream the butter and sugar on medium-high speed, until very creamy and pale, at least 3 to 4 minutes.  You want to beat it until the sugar starts to melt into the butter, and not feel so grainy.  Add the egg yolks, one at a time, mixing well after each addition.  Add the vanilla and mix, scraping down the sides of the bowl.  Turn mixer to low speed and add 1/3 of the flour, 1/2 of the buttermilk, 1/3 of the flour, the second 1/2 of the buttermilk, and ending with the last 1/3 of the flour.  Stop mixing when the flour is just almost incorporated.  Do not overmix!!
4. Add the egg whites to a medium bowl.  Using a hand whisk, whisk the egg whites until they are soft peaks.  Add about 1 cup of the cake batter to the egg whites.  Using a spatula, gently fold the two together.  Add the egg whites to the bowl of batter.  Using a spatula, gently fold them in by running the spatula down the sides of the bowl, under the batter, and back up the sides of the bowl.  Repeat until the batter is equally blended.
5. Divide the batter equally between the cake pans.  Using an offset spatula, smooth the top of the batter in each pan.  Do not tap the pans on a counter to remove air bubbles, like you normally would in a regular cake.  This will deflate the cake batter.
6. Bake the cakes in the center of the oven until a toothpick inserted in the centers comes out clean, 30 to 35 minutes, rotating the pans halfway through.
7. Transfer cakes to a wire rack.  Allow the cakes to cool in the pans for about 10 minutes.  Invert the cakes onto a wire rack.  Peel off the wax or parchment paper.  Allow cakes to cool completely before frosting.

FROSTING

1. In a small bowl, whisk together the cocoa and 1/4 cup plus 2 tablespoons of warm water until the cocoa is dissolved.  Set aside.
2. In a large bowl, using an electric mixer, beat the butter, powdered sugar, salt, and vanilla together on medium speed until the mixture is pale and fluffy.  Gradually beat in the melted chocolate, followed by the cocoa.  Beat until well combined.
3. Allow the frosting to stand for 30 minutes before using.
4. Using a serrated knife or cake leveler, trim the tops of the cakes to level them if desired.
5. Frost the cake and serve.  Note:  I don’t recommend refrigerating this cake if you’re using this frosting recipe.  The frosting becomes very firm when chilled.