Preheat the oven to 350°F and grease a 13x9-inch pan.
Combine the graham cracker crumbs with the powdered sugar and some of the butter.
Add more butter until you have a sandy texture.
Press the mixture into the bottom of the greased pan.
Bake for 5 minutes or until lightly golden at the edges.
Let the crust cool and set.
Arrange the chocolate bars on top and bake for 3 minutes.
Let it cool for a couple of minutes.
Arrange the marshmallows on top.
Broil for a minute or so, keeping an eye on them the whole time because they can scorch and burn fast.
Let them cool down, then cut and serve.