1. Heat oven to 350°F. Grease 8-inch square baking pan.
2. Beat butter and sugar until well blended 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 half of dough in prepared pan. Bake 15 minutes.
3. Break chocolate bars into pieces; sprinkle over baked layer. Top with marshmallows; scatter bits of remaining dough over marshmallows, forming top layer. Bake 10 to 15 minutes or just until lightly browned. Cool completely in pan on wire rack. Cut into bars. Makes 16 bars.