1. Heat oven to 350°F. Grease 15-1/2x10-1/2x1-inch jelly-roll pan.
2. Beat butter and sugar until light and fluffy in large bowl. Add egg and vanilla; beat well. Stir together flour, graham cracker crumbs, baking powder and salt; add to butter mixture, beating until blended. Press dough into prepared pan. Bake 15 minutes.
3. Meanwhile prepare S'MORE TOPPING. Evenly spread topping mixture over partially baked cookie crust. Bake additional 15 to 20 minutes or until marshmallows are lightly browned. Cool completely in pan on wire rack. Cut into bars.
4. Serve as bars or warm bars slightly and serve with ice cream. Makes 12 servings.
3 cups (about 11 whole crackers) graham cracker pieces (1/2 to 1-inch pieces)
4 cups (about 12 (1.55 oz.) bars) HERSHEY'S Milk Chocolate Bar pieces
2-1/2 cups miniature marshmallows
1 jar (7 oz.) marshmallow creme
Carefully mix together graham cracker pieces, chocolate bar pieces, miniature marshmallows and marshmallow creme in large bowl.