1-Preheat the oven to 350°F (175°C). Grease an 8-inch square baking pan or line it with parchment paper.

2-In a large bowl, cream together the butter and sugars until light and fluffy. Beat in the egg and vanilla extract.

3-In a separate bowl, whisk together the flour, graham cracker crumbs, baking powder, and salt. Gradually add the dry mixture to the butter mixture and mix at low speed until combined.

4-Divide the dough in half. Press half of the dough into an even layer on the bottom of the prepared pan. Place chocolate bars over the dough in a single layer, breaking them if needed to fit. Spread the marshmallow fluff evenly over the chocolate layer.

5-Form the remaining dough into small disks, flatten them, and arrange them evenly over the marshmallow fluff to cover completely.

6-Bake for 25 to 35 minutes or until the top is lightly browned. Allow to cool completely before cutting into bars.