Preheat oven to 350°F (175°C) and line an 8×8 or 9×9 inch baking pan with parchment paper. In a medium bowl, mix melted butter and sugar. Whisk in eggs, then cocoa powder. Add flour, salt, and baking powder. Mix until combined. Spread brownie batter evenly into pan. In another bowl, cream softened butter with brown and granulated sugar. Add egg and vanilla extract, mix well. Stir in flour, baking soda, and salt. Fold in chocolate chips and optional food coloring or sprinkles. Spoon cookie dough in dollops over brownie batter. Lightly press down with a spatula to spread. Don’t cover completely. Bake for 25–30 minutes, or until a toothpick comes out with moist crumbs. Let cool before slicing. Decorate with candy eyes or sprinkles if desired.