1. Gently melt chocolate, butter and milk together in a bowl over a saucepan of simmering water.
2. Beat eggs sugar with an electric mixer until thick and pale. Gently fold in the chocolate mixture. Sift flour and cocoa over the batter and fold gently to combine.
3. Pour mixture into buttered, side and base lined 23cm cake pan and bake at 160°C (150°C fan forced) for 45-50 minutes or until cooked when tested with a skewer. Cool for 5 minutes in pan before turning onto a wire rack to cool completely. Slice cake in half using a serated knife and refrigerate until chilled.