Preheat the oven to 150°C and line 2 baking sheets with baking paper. Cream together the butter, brown sugar and white sugar until pale and fluffy.
Add the eggs one by one, beating well after each addition. Add the flour, cocoa powder and baking powder and mix well. Roll tablespoons of the mixture into balls, arrange them on the lined baking sheets and flatten them slightly.
Bake for 15 minutes or until they just begin to change colour on the bottom. Turn out onto cooling racks and leave to cool completely.
Chocolate Filling Melt the chocolate in a bowl over hot water or in the microwave oven. Remove from the heat, add the sour cream and stir until well blended.
Spread half the cookies with the filling and sprinkle with the nuts. Cover with the remaining cookies.