Preheat oven to 350º F. Line a circular 10 inch cake tin (3 inches tall) with grease proof or other non-stick paper and grease the tin. (Please note that the cake will rise to 3 inches and collapse somewhat when cooled. If your cake tin is less than 10 inches wide and 3 inches tall we recommend that you use two cake tins.)
Break the chocolate into small pieces and melt it with butter over hot water.
Beat the eggs with sugar, mix with flour, cocoa powder, baking powder and vanilla extract.
Slowly fold in the melted butter and chocolate and the sour cream.
Bake at 350º until a wooden pick inserted in center comes out clean, approximately 40 to 50 minutes (if using 2 cake tins 20-30 minutes may be sufficient).
Cool the cake completely. When it has obtained room temperature place it in the refrigerator for at least 30 minutes before removing the cake from the tin (the cake is sticky and difficult to cut when it is warm!) Remove the crusted surface on the top of the cake, and cut in half, horizontally.
FROSTING: Heat 2/3 cup of heavy cream or whipping cream in a sauce pan. Remove from heat, add 9 oz of finely chopped dark semisweet chocolate, stir until smooth, and let it cool until in thickens.
Use one 1/3 of the frosting between the two layers, 1/3 on top, and the rest around the cake. Put the cake into the fridge for one hour or more to harden the frosting.
This cake should have room temperature when served.