Directions

1. 1

   Preheat oven to 350°F. Butter an 8x8-inch baking dish and line it with two sheets of parchment paper, allowing the edges to overhang slightly on all sides, then butter the parchment. (Alternatively, you may simply butter the baking dish without lining if you prefer.)

2. 2

   In a medium bowl, whisk together the flour, graham cracker crumbs, baking soda, and salt for about 20 seconds. Set the dry mixture aside.

3. 3

   Using an electric stand mixer fitted with the paddle attachment, cream together the softened butter, packed brown sugar, and granulated sugar until the mixture is light and fluffy.

4. 4

   Add the egg and vanilla extract to the mixer bowl and beat until fully incorporated.

5. 5

   With the mixer on low speed, gradually add the dry ingredients to the wet mixture, stirring just until combined. Do not overmix.

6. 6

   Divide the dough in half. Press one half of the dough evenly into the bottom of the prepared baking dish, creating a firm base layer.

7. 7

   Arrange the milk chocolate bars side by side directly over the dough layer on the bottom of the pan.

8. 8

   Spread the marshmallow creme evenly over the chocolate layer, covering it completely.

9. 9

   Pinch off golf-ball-sized pieces of the remaining dough and flatten each piece into a small shingle shape. Arrange these dough shingles side by side and slightly overlapping over the marshmallow layer, working carefully to minimize any exposed marshmallow.

10. 10

    Bake in the preheated 350°F oven for 27 to 30 minutes, until the top layer of dough is light golden brown.

11. 11

    Remove the bars from the oven and cool completely on a wire rack before cutting. If you lined the pan with parchment, use the overhanging edges to lift the entire slab from the pan before cutting into squares.

12. 12

    For best results, cut the bars just before serving, as the marshmallow filling may become slightly runny at room temperature. Store any leftovers in an airtight container.