Preheat your oven to 350°F (175°C) and line a 9x13-inch baking pan with aluminum foil, spraying it with nonstick spray.

Whisk together flour, graham cracker crumbs, baking powder, and salt in a bowl. In another bowl, beat together butter and brown sugar until light and fluffy, then add eggs and mix.

Gradually add dry ingredients to the wet mixture, stirring until just combined to keep the bars soft and chewy.

Press half of the dough into the bottom of the prepared pan to form a base layer.

Spread marshmallow topping evenly over the dough, then sprinkle milk chocolate chips on top.

Crumble the reserved dough over the top, allowing some marshmallow and chocolate to be visible.

Bake for 30–35 minutes, until the top is golden brown and edges are slightly firm but center is gooey.

Let the bars cool in the pan on a wire rack for 1 hour, then lift out using the foil and cut into squares.