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

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 for about 3 minutes. Add the egg and vanilla extract and beat until combined. Scrape down the sides and bottom of the bowl as needed.

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

Press/spread ⅔ of the graham cookie dough into the prepared pan. 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.

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.