Preheat oven to 350°F (175°C) and grease an 8x8 inch square baking pan.

In a large bowl, cream together softened butter, granulated sugar, and brown sugar until light and fluffy.

Beat in the egg and vanilla extract until well 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, mixing until a stiff dough forms.

Divide the dough into two equal portions.

Press one half of the dough into the bottom of the prepared baking pan in an even layer.

Place the milk chocolate bars side-by-side on top of the dough layer, breaking them as needed to fit.

Spread the mini marshmallows evenly over the chocolate bars.

Crumble the remaining dough over the marshmallow layer, pressing down lightly to flatten.

Bake for 30 minutes, or until the top is golden brown and the marshmallows have puffed.

Allow the bars to cool completely in the pan before cutting into squares.