Crust: Grease two 9inch cake pans and line each with a circle of parchment or wax paper; set aside. In a food processor or with a rolling pin, crush Oreos. In a bowl, combine crumbs with melted butter; pat into the bottom of one prepared pan. Bake at 350 degrees for 5 minutes. Set aside. Cake: In a bowl, combine coarsely crushed Oreo cookies, flour, baking powder and baking soda; set aside. In another bowl, beat butter and sugar until fluffy. Beat in egg yolks, sour cream, melted chocolate and vanilla. Add water all at once and stir to combine. Fold in dry ingredients; beat at medium speed for 1 minute. Beat egg whites until soft peaks form. Fold into batter by hand. Divide the batter between the two pans; one has a cookie bottom, one does not. Bake at 350 degrees until cake just starts to spring back when touched, 15 to 25 minutes. The bottom layer with the cookie crust will take about 5 minutes longer to bake. Remove cake layers from pans and cool in the refrigerator. Filling: In a bowl, combine butter, confectioners sugar and vanilla. Add enough milk to make a spreading consistency. Place cooled layer with cookie crust bottom on a plate. Reserving some filling for garnish, spread remaining filling over this layer. Frosting: In a bowl, combine butter, confectioners sugar, cocoa and vanilla. Add enough milk to make a spreading consistency. Place remaining layer over filling, then spread on frosting. Garnish: Press additional crushed Oreo cookies around the sides of the cake. Use a pastry bag to pipe reserved white filling on top to make little clouds. Press a miniature Oreo or onefourth of an original Oreo into each puff. Makes 12 servings.