FROSTING: Melt remaining (1/2 c) butter. Add 1/2 c 10x sugar and remaining (1/2 c) cocoa. Stir with a wire whisk over low heat until smooth; cool. Beat remaining (1 1/2 c) heavy cream; add remaining (1 1/2 c) 10x sugar until soft peaks form. Add cooled chocolate mixture; beat until stiff. Fold in remaining (3/4 c) sour cream. CONSTRUCTION: Build layers from the bottom up as follows: Cake, 1/3 the filling, cake, 2/3 c frosting. Repeat above twice, for total of six layers. Put remaining frosting on sides. Chill two hours before serving. Note: The cakes must be well-chilled before assembly, or they will disintegrate under their own weight during assembly. If the cake tops rose during baking, the layers will not stack well. You will need to slice the excess off after chilling, but before splitting into layers.