Preheat the oven to 350 degrees. Line a 13x9-inch baking pan with foil, allowing the foil to come up and over the shorter sides (this creates 'handles" for easy removal later).

Arrange the graham crackers in the bottom of the prepared pan, trimming some to make sure they all fit in a single layer.

Prepare the brownie batter according to the package instructions. Pour the batter over the graham crackers and smooth the surface. Bake as directed on the package (for me that was 21 to 23 minutes), until a wooden pick inserted near the center comes out sticky, or with little moist bits clinging to it.

Remove the pan from the oven and preheat the broiler. Top the brownies with the marshmallows, covering the brownie layer (if necessary, add marshmallows to make sure you completely cover the brownies). Place the brownies under the broiler and cook for 15 to 45 seconds, until the marshmallows are golden brown - stay close - this happens fast!

Cool the brownies completely before cutting, about 30 to 60 minutes. Use the foil "handles" to transfer the brownies to a cutting board.

Use a wet knife to cut the brownies into squares. Store leftovers in an airtight container at room temperature or in the refrigerator.