Preheat oven to 350°F (177°C). Line an 8-inch square baking pan or 9-inch square baking pan with parchment paper, leaving enough overhang on the sides to easily remove the bars when they have cooled. Set aside.

In a large bowl using handheld or stand mixer fitted with a paddle attachment, cream the butter and brown sugar together on medium-high speed, about 3 minutes. Add the egg and vanilla extract and beat until combined. Scrape down the sides and up the bottom of the bowl as needed. Mixture may appear curdled and that’s ok.

Whisk the flour, graham cracker crumbs, baking powder, and salt together in a medium bowl. Pour the flour mixture into the wet ingredients and then beat on medium speed until combined.

Press/spread 2/3 of the graham cookie dough into prepared pan. It will be a thin layer and the dough is sticky, so use your hands if needed. Spread the marshmallow creme evenly on top. Layer chocolate chips evenly on top. Mold the remaining cookie dough into flat pieces and layer on top of the chocolate chips; some chocolate chips/marshmallow will be exposed.

Bake the bars for 25-30 minutes or until the top is lightly golden brown. Remove from the oven and place the pan on a wire rack. Cool bars completely.

Lift the bars out using the parchment paper overhang on the sides. Cut into squares.

Cover and store leftover bars at room temperature for up to 1 week.