Preheat oven to 350°F.
Generously butter and flour two 9-inch cake pans.
In a saucepan, slowly melt chocolate and add ½ cup buttermilk. Beat with a whisk, stirring over low heat until mixture is smooth.
Add 1 cup sugar and 1 egg yolk. Stir constantly for 3 minutes or until the custard is thick and smooth. Cool.
Cream butter and brown sugar well. Add 2 egg yolks, one at a time.
Resift cake flour with baking soda and salt. Add to butter mixture in 3 parts, alternating with ¾ cup buttermilk.
Stir the cooled custard and vanilla into the batter.
Beat 3 egg whites until they are stiff but not dry. Fold into batter.
Pour into prepared pans.
Bake for 25-30 minutes or until cake just begins to leave the sides of the pans or centers spring to touch. Completely cool in pans before removing.
Mix all the above ingredients together well and spread frosting between layers as well as on top and sides of cake.