Preheat oven to 350° F.
Line a 9 inch cake pan with grease proof or other non-stick paper and grease the pan.
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 350° for 50 minutes or until a wooden pick inserted in center comes out clean.
Cool the cake. Remove the crusted surface on the top of the cake, and cut in half, horizontally.
Heat 2/3 cups of heavy cream in a sauce pan.
Remove from heat, add 9 ounces of finely chopped dark chocolate (40-45% cocoa), and stir until smooth.
To test it, transfer a Tablespoon of the frosting into a chilled cup and place in the fridge for 15 minutes. If it is too thin, add some more melted chocolate. If it is too thick, add a few Tablespoons of cream, stir until smooth, and test it again.
This cake must be at room temperature when served to have soft frosting.