Preheat oven to 350º. Grease and flour lightly 2 – 8” round cake pans.
In a medium bowl sift together the flour, cocoa powder, baking powder an salt. Set aside. Mix together in a measuring cup, the milk and the vanilla, set aside.
In a large bowl, cream the butter and sugar until light and fluffy with an electric beater. Beat in the egg yolks, one at a time. Add ½ cup flour of the flour mixture, ½ cup of the milk mixture and beat until well blended. Repeat with the flour mixture and vanilla mixture 3 more times until well blended and smooth. Set aside.
With an electric beater, beat the egg whites until they are firm enough to stand in soft peaks. Scoop the chocolate mixture over the egg whites and with a rubber spatula begin to gently but thoroughly incorporate by folding together.
Pour the batter into the prepared cake pans, dividing equally and smoothing the tops. Bake in middle of oven for 45 min or until cake begins to spring back when touched.
Remove to wire racks to cook before frosting.
Drop the chocolate into the top of a double boiler and stir until chocolate melts. Remove from heat and beat in the sour cream and vanilla.
Set one cake upside down on a serving platter and spread ½ cup of the frosting over it. Place the 2nd cake on top and coat the top and sides with the remaining frosting.