Preheat oven, grease baking pan, dust with flour; tap out excess.

Mix together flour, cocoa, baking powder, baking soda and salt. Set aside.
Beat together butter, white sugar and brown sugar at medium speed until light and fluffy. Add egg yolks, one at a time, beating well after each addition. Beat in milk, chocolate and vanilla. Alternately beat flour mixture and sour cream into chocolate mixture.

Using clean beaters, beat egg whites at high speed until stiff, but not dry, peaks form. Fold egg whites into batter. Spoon batter into prepared pan; smooth top.
Bake until a toothpick inserted in center comes out clean. Cool for 5 minutes.

Then turn out cake onto wire rack to cool completely.

Frosting

In top of double boiler set over simmering (not boiling) water, melt chocolate, stirring until smooth. Cool for 10 minutes. Beat butter at medium speed until smooth. Beat in eggs, one at a time, beating well after each addition. Beat in chocolate, vanilla and sugar until thick and fluffy. Spread frosting on top and sides of cake. For garnish, using a pastry bag fitted with a 1 inch star tip, pipe whipped cream rosettes onto the cake as a border. Pipe a decorative edge around bottom of cake. Place cherries on top of whipped cream.