Preheat your oven to 350°F. Line a 13x9-inch baking pan with parchment paper or foil and lightly coat with nonstick spray.

Arrange graham cracker sheets in a single layer on the bottom of the pan, trimming as needed to fit.

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

Beat in the egg and vanilla until fully blended.

In a separate bowl, whisk together the flour, baking soda, and salt. Gradually add the dry ingredients to the wet mixture, stirring until combined.

Fold in the mini marshmallows until evenly distributed throughout the dough.

Drop spoonfuls of dough across the graham cracker layer and gently spread it out as evenly as possible without shifting the crackers.

Press chocolate bar pieces gently into the top of the dough.

Bake for 18 to 20 minutes, or until the cookie layer is golden and set.

Cool completely in the pan before lifting out and slicing into bars.