In a large bowl, cream butter and sugar until light and fluffy. Beat in egg and vanilla. Combine the flour, cracker crumbs, baking powder and salt; gradually add to creamed mixture. Set aside 1/2 cup for topping. Press remaining mixture into a greased 9-in. square baking pan. Place candy bars over crust; spread with marshmallow creme. Crumble remaining graham cracker mixture over top. Bake at 350° for 25-30 minutes or until golden brown. Cool on a wire rack. Cut into bars. Store in an airtight container. Yield: 1-1/2 dozen.