Heat oven to 350°F. Line 8 x 8 square pan with parchment paper or grease with oil or non-stick spray.In a bowl, stir together the marshmallows, chocolate chips, and nuts. Set aside.In another bowl, mix butter and sugar together. Add eggs and extract. Stir well.Add flour, cocoa & baking powder. Stir on top of bowl to mix together. Then mix in with butter & sugar.Pour batter into pan. Bake for 20 minutes.Carefully pull out pan and spread topping on top.Put pan back in the oven to bake another 5 minutes.