In a large bowl, cream together butter and sugars until light and fluffy. Beat in egg and vanilla. Add flour, graham cracker crumbs, baking powder, and salt to butter mixture. Mix on low speed until combined. Divide dough in half.
Press half of dough into bottom of prepared pan. Place chocolate bars over dough. Alternatively, use chocolate chips and layer no more than 1/4" thick. Spread marshmallow creme over chocolate. Place remaining dough on top of creme.
Bake 30-35 minutes. Cool completely before cutting into bars.