1/3 cup butter
1/3 cup white sugar
1/3 cup brown sugar
1/4 cup cocoa
1/3 cup flour
1/2 tsp baking powder
1/2 tsp vanilla extract)
1/3 cup chopped walnuts (omit for a nut-free version)
1/3 cup chocolate chips (increase to 2/3 cup if making a nut-free version)
Preheat oven to 350. Grease an 8-inch square baking dish. Put butter into the dish and microwave for a few seconds, until it has melted. Add white and brown sugars and mix well. Add the cocoa and egg and continue mixing well. Sift in the flour and baking powder. Mix in the vanilla extract and chopped nuts, if using. Finally, add half the chocolate chips and mix in gently, then sprinkle the other half on top.
Place the dish in the oven for 35-40 minutes, or until a toothpick inserted in the center comes out clean. Remove from the oven, cut into squares and allow to cool.