Beat sugar, butter and egg in large bowl at medium speed until light and fluffy. At low speed, beat in flour, baking powder and salt until blended. Spread batter in greased 9x13" pan. Bake at 350 ° for 10 to 12 minutes, until set. Sprinkle chocolate over batter. Top with marshmellows and peanuts. Bake 10 to 15 minutes, until marshmellows are lightly browned. Cool and cut into bars.