1. Preheat the oven to 350°F (177°C) and line an 8×8 or 9×9-inch pan with parchment paper for easy removal.
2. In a mixing bowl, cream the butter and brown sugar on medium-high speed for about 2 minutes until smooth.
3. Add the egg and vanilla extract; beat until well combined.
4. In a separate bowl, whisk together the flour, graham cracker crumbs, baking powder, and salt.
5. Gradually combine the dry ingredients with the wet ingredients until a soft dough forms.
6. Press two-thirds of the dough into the bottom of the prepared pan.
7. Spread the marshmallow creme evenly over the dough, then sprinkle with chocolate chips.
8. Flatten small pieces of the remaining dough and dot them over the top; marshmallow and chocolate will peek through.
9. Bake for 25-30 minutes or until the top is lightly golden brown.
10. Allow to cool completely in the pan before removing and cutting into squares for serving.