1. Preheat oven to 350ºF. Line a 9-by-13-inch baking pan with foil, leaving a 2-inch overhang on 2 sides. Mist foil with cooking spray.
2. In a bowl, combine flour, graham cracker crumbs, baking powder and salt; mix well. In a large bowl, using an electric mixer on medium speed, beat butter and sugar until light, about 2 minutes. Beat in eggs until well combined. Reduce mixer speed to low and slowly beat flour mixture into butter mixture. Divide dough in half. Press half of dough into pan until dough is uniform and flat. Spread with marshmallow topping and sprinkle chocolate chips on top. Scatter remaining dough over in clumps. Don't worry about covering the top; marshmallow and chocolate chips should peek through.
3. Bake until golden brown, 30 to 35 minutes. Place pan on a wire rack to cool completely. Remove from pan, peel off foil and cut into bars.