Start by preheating your oven to 350°F. Line an 8×8 inch baking pan with parchment paper, leaving some overhang on the sides.
In a medium bowl, mix the graham cracker crumbs, melted butter, and granulated sugar until it looks like wet sand. Press this mixture firmly into your prepared pan using the bottom of a measuring cup. You want it pretty firmly packed or it’ll crumble when you cut the bars.
Pop the crust in the oven for about 8-10 minutes, just until it starts to smell toasty and turns a slightly deeper golden color. This short bake helps the crust hold together better.
While the crust is still hot, dollop the peanut butter all over the surface. Give it about 2 minutes to start melting from the heat of the crust, then use an offset spatula or the back of a spoon to spread it into an even layer.
Sprinkle the milk chocolate chips evenly over the peanut butter layer, then top with mini marshmallows. Try to get the marshmallows in a single layer – they’ll expand as they toast.
Switch your oven to broil and position a rack about 6 inches from the heat. Place the pan under the broiler for about 1-2 minutes, watching constantly. You’re looking for the marshmallows to puff up and turn golden brown.
Once the marshmallows are toasted, take the pan out and let it cool on a wire rack for about 30 minutes. The chocolate chips will be fully melted underneath those marshmallows.
For the finishing touch, melt the semi-sweet chocolate chips in the microwave in 20-second bursts, stirring between each burst until smooth. Using a spoon, drizzle the melted chocolate over the marshmallow layer in a zigzag pattern.
Pop the whole thing in the fridge for at least 2 hours or until completely set. The bars will be much easier to cut when chilled. Use the parchment overhang to lift the entire block out, then cut into squares with a sharp knife.