Preheat oven to 350°F (180°C). Combine flour, baking powder and salt in medium bowl; set aside.
Melt butter in large saucepan over low heat; remove from heat. Stir in cocoa. Beat in brown sugar, sugar, eggs and vanilla. Stir in dry ingredients and walnuts.
Spray or butter a 9-inch (23 cm) square pan. Spread batter in pan. Top batter with half of your Halloween candy (some of this will sink into the brownies while baking). Bake in preheated 350°F (180°C) oven for 30-35 minutes. When brownies are 5 minutes away from being done, add the rest of your Halloween candy. Bake for an additional 5 minutes. Remove from oven and cool completely.