Preheat oven to 350 degrees. Lightly grease an 8-inch square baking pan.
In a large bowl, beat butter and sugar with an electric mixer until creamy. Add eggs and vanilla; beat until combined.
In a small bowl, combine flour, graham cracker crumbs, baking powder, and salt. Add to butter mixture, and beat until combined.
Turn the baking pan upside down and place a piece of waxed paper on top. Press half of the dough onto the waxed paper, using the bottom of the pan as a guide for the size. Set aside. Turn pan back over.
Press the other half of the dough evenly in the bottom of the pan. Spread marshmallow cream over the dough, but don’t go all the way to the edge. Lay the chocolate bars on top of the marshmallow creme, breaking apart as needed to create an even layer of chocolate. Take the preformed dough that you set aside earlier and invert onto the top of the chocolate and peel away the waxed paper. Spread gently to the edges if needed.
Bake for 25-30 minutes (mine was 27). Cool completely before cutting into squares.
Don't want to run to the store for marshmallow creme? Here's how to make your own: Over low heat melt 7 ounces (about 4 1/2 cups) of miniature marshmallows and 3 tablespoons of light corn syrup. Let cool in a bowl sprayed with cooking spray. Your homemade marshmallow creme will slide right out when you are ready to add it to the recipe.
I highly recommend an inexpensive kitchen scale for so many recipes. It's super handy when you have to portion out dough or batter, like splitting cake batter into pans.