Heat the oven to 350°F. Line an 8x8-inch baking pan with parchment paper, leaving some hanging over two sides.
In a large bowl, beat the butter, granulated sugar and brown sugar until light and fluffy. Beat in the egg and vanilla.
In a separate bowl, whisk the flour, graham cracker crumbs, baking powder and salt. Stir into the butter mixture until a soft dough forms.
Press a little more than half of the dough evenly into the bottom of the pan.
Lay the chocolate bars over the dough in a single layer, breaking them to fit. Spread the marshmallow creme over the chocolate.
Flatten pieces of the remaining dough in your hands and lay them over the marshmallow. It’s fine if there are small gaps.
Bake for 30–35 minutes, until lightly golden on top.
Cool completely in the pan, at least 1 hour. Lift out by the parchment and cut into 16 bars.
