Preheat oven to 350 °F (175 °C). Line an 8×8-inch baking pan with parchment paper, leaving an overhang for easy removal.

In a bowl, whisk together flour, graham crumbs, sugar, baking powder, and salt. Pour in melted butter and stir until mixture clumps together.

Press about two-thirds of the mixture firmly into the bottom of the prepared pan to form the crust.

Sprinkle chocolate chips over crust, then evenly distribute marshmallows on top.

Sprinkle the remaining crumb mixture over the marshmallows and chocolate to form a streusel-like topping.

Bake for 20–25 minutes, or until top is golden and marshmallows are gooey and slightly puffed.

Remove from oven and let cool completely in the pan—about 30 minutes—to allow the bars to set.

Using the parchment overhang, lift the slab from the pan and cut into 12 squares. Serve at room temperature (warm is optional but messy).