In a large bowl, cream butter and sugar until light and fluffy. Add egg and vanilla; beat for 2 minutes. Combine buttermilk and sour cream. Combine the flour, cocoa, baking soda and salt; add to creamed mixture alternately with buttermilk mixture, beating well after each addition.
Pour into two greased and waxed paper-lined 9-in. round baking pans. Bake at 350° for 20-25 minutes or until a toothpick inserted in the center comes out clean. Cool for 10 minutes; remove from pans to wire racks to cool completely.
In a heavy saucepan, melt chocolate with cream over low heat. Remove from the heat; stir in vanilla. Transfer to a small mixing bowl; chill until slightly thickened, stirring occasionally. Beat on medium speed until light and fluffy. Chill until mixture achieves spreading consistency.
For ganache, in a heavy saucepan, melt chocolate and butter. Gradually add cream; heat until just warmed. Chill until slightly thickened.
Place one cake layer on a serving plate; spread with filling. Top with remaining cake layer. Slowly pour ganache over top of cake. Refrigerate until serving.