Preheat oven to 350°F (177°C).
Line an 8-inch or 9-inch square baking pan with parchment paper, leaving overhang on the sides for easy removal.
In a large mixing bowl, beat the softened butter and brown sugar together on medium-high speed for about 3 minutes until light and creamy.
Add the egg and vanilla extract and mix until combined.
In a separate bowl, whisk together the flour, graham cracker crumbs, baking powder, and salt.
Gradually add the dry ingredients to the wet ingredients and mix until a soft dough forms.
Press approximately two-thirds of the dough evenly into the prepared baking pan.
Spread the marshmallow creme over the dough layer.
Sprinkle the chocolate chips evenly across the marshmallow layer.
Flatten pieces of the remaining dough and place them over the chocolate chips. Some chocolate and marshmallow will remain visible.
Bake for 25–30 minutes or until the top is lightly golden brown.
Remove from the oven and allow the bars to cool completely in the pan.
Use the parchment paper overhang to lift the bars out of the pan.
Cut into squares and serve.