- Preheat the oven to 350°F. Grease an 8-inch square baking pan.
- In a large mixing bowl, cream together butter, brown sugar, and granulated sugar until light and fluffy, about 3 minutes. Beat in egg and vanilla.
- In a small bowl, whisk together flour, graham cracker crumbs, baking powder, and salt. Add to butter mixture and mix at low speed until combined.
- Divide the dough in half and press half of the dough into an even layer on the bottom of the prepared pan.
- Place the chocolate bars over the dough.
- Evenly place dollops of marshmallow fluff over top of the chocolate layer. If possible, spread smooth.
- On a well-floured surface, roll out the remaining graham cracker dough into an 8”x 8” square (approximately). Place over the marshmallow layer and press down to form a somewhat even layer.
- Bake for 30-35 minutes, until lightly browned. Cool completely before cutting into bars.
Makes…16 cookie bars