Preheat oven to 350 degrees. Grease an 8 inch square baking pan with nonstick cooking spray. Add butter and both sugars to a stand mixer with the paddle attachment. Beat until light and fluffy, 2-3 minutes.
Add in egg and vanilla and mix until combined.
Meanwhile in a medium bowl combine flour, graham cracker crumbs, baking powder and salt.
Add the dry ingredients to the stand mixer and mix just until combined.
Press half the dough into the prepared pan and bake for 15 minutes.
Remove from the oven but don't turn the oven off. Layer the chocolate bars ontop of the pan and sprinkle with marshmallows.
Spread the remaining dough on top. (It's ok if it does not cover the marshmallows)
Bake for 15 minutes or until just lightly browned and the bars look set. Allow the bars to cool completely on a wire rack before cutting. They are very crumbly and hard to cut if you do it while they are warm.