Preheat oven to 160°C (320°F)
Grease a deep 25cm (10") round cake pan, line base with baking paper.
Beat butter and caster sugar in a large bowl with an electric mixer until light and fluffy. Beat in eggs, one at a time.
Melt chocolate in a heat proof bowl, over a small pot of simmering water.
Stir in half the sifted flour, baking powder and cocoa, along with half the sour cream.
Stir in remaining dry ingredients and sour cream, along with the melted chocolate. Mix gently until smooth.
Pour into your prepared cake tin, and bake for 1 hour 20 minutes (or until cooked when tested with a skewer).
Stand cake for 10 minutes before turning onto a wire rack to cool. Ice when cold with chocolate fudge frosting.