First melt the butter and the chocolate in the microwave (or in a water bath). Add sugar and stir well with a hand mixer. First beat an egg, stir through, then beat the next egg, stir through. Add flour and baking powder and stir through. Finally, stir in the chopped walnuts and pour into a greased baking pan* (with a tiny bit of flour). Bake at 160 degrees for 25 minutes. Attention: Do not bake too long, so that the cake stays soft inside and does not dry out! Cool a little before cutting and sprinkle with powdered sugar.
A port wine fits to it [*Note: medium cake springform].
Good luck!