Preheat your oven to 350°F (175°C). Grease and flour three 9-inch round cake pans or line the bottoms with parchment paper.
In a small saucepan, melt the chocolate in boiling water, stirring until smooth. Let cool to room temperature.
In a large bowl, cream together the butter and sugar until light and fluffy, about 2–3 minutes.
Add the egg yolks, one at a time, beating well after each addition. Stir in the cooled chocolate and vanilla extract.
In a separate bowl, sift together the cake flour, baking soda, and salt. Add the dry ingredients to the chocolate mixture alternately with the buttermilk, beginning and ending with the dry ingredients. Mix until just combined.
In another bowl, beat the egg whites until stiff peaks form. Gently fold the egg whites into the batter.
Divide the batter evenly among the prepared pans. Bake for 25–30 minutes, or until a toothpick inserted into the center comes out clean.
Cool the cakes in the pans for 10 minutes, then remove to a wire rack to cool completely.