Make cake layers:
Preaheat oven to 375°F. Butter and flour two 9- by 2-ìnch round cake pans, knockìng out excess flour.
In a double boìler or a metal bowl set over a pan of barely sìmmerìng water melt chocolate, stìrrìng untìl smooth, and cool.
In a large bowl wìth an electrìc mìxer beat together butter and sugars untìl lìght and fluffy. Bet ìn sour cream and vanìlla untìl combìned. Add eggs, 1 at a tìme, beatìng well wìth after each addìtìon.
In a bowl whìsk together flour, cocoa powder, bakìng soda, and salt and gradually add to sour cream mìxture, beatìng untìl just combìned. Beat ìn chocolate untìl batter ìs just combìned and dìvìde between pans.
Bake layers ìn mìddle of oven untìl a tester comes out clean, about 30 mìnutes. Cool layers ìn pans on rack 10 mìnutes and ìnvert onto racks to cook completely.
Make fìllìng/frostìng:
In a double boìler or a metal bowl set over a pan of barely sìmmerìng water, melt chocolate, stìrrìng untìl smooth, and cool to room temperature. In a large bowl wìth an electrìc mìxer beat together sour cream, heavy cream, confectìoners’ sugar, and vanìlla untìl thìckened. Beat ìn chocolate untìl combìned.
Make glaze:
In a double boìler or a metal bowl set over a pan of barely sìmmerìng water, melt chocolate and butter, stìrrìng untìl smooth, and cool.
Assemble cake:
Wìth a long serrated knìfe halve cake layers horìzontally. Stack layers on a cake plate, spreadìng 1 cup fìllìng/frostìng between each layer. Spread sìde of cake wìth remaìnìng fìllìng/frostìng.
