Preheat the oven to 400°F. Lightly grease a 9-by-13-inch baking pan with nonstick spray. Next, line the pan with parchment paper, allowing parchment to hang over the edges.

CRUST: Twist the tops off the Oreo cookies. Set aside the bottoms (with the cream).

In a food processor, add the Oreo cookie tops and the graham crackers; pulse until finely ground.

Transfer the graham cracker/Oreo cookie crumbs to a medium bowl. Add the sugar and salt to the graham-cracker crumbs and stir to combine. Add the melted butter and stir. Pour the mixture into the prepared 9x13 pan. Press the crust with your fingers, to make an even layer.

Bake for 8 to 10 minutes, or until the edges are lightly golden. Remove from oven and allow to cool for 5-8 minutes.

TOPPING: On top of the crust, lay down the chocolate bars, leaving about ¼ inch of crust uncovered around the edges. Next, arrange the 24 cookie bottoms, cream-side up, on top of the chocolate bars. Place the marshmallows on top of the Oreo cookie bottoms. The marshmallows will increase in size as they bake.

Bake for 10-12 minutes, or until the chocolate is soft and the marshmallows are golden brown on top.

Serving gooey bars: Slice and serve immediately.

Serving firm bars: Allow the bars to cool when removing from the oven, for at least 20 minutes.

If serving later, reheat individual servings for a gooey experience!