Place the graham crackers in a resealable bag and hit with a rolling pin until they're in small pieces and crumbs but not powdered.
Preheat the oven to 350 degrees F, and place a baking stone or inverted baking sheet on the center rack to heat.
In a medium-sized bowl, combine the cracker crumbs, butter, sugar, and salt together using your hands or a silicone spatula. Knead and press the mixture until it's evenly combined. Transfer the mixture to the slab pie pan and press evenly into the bottom, but not up the sides of the pan. Use the bottom of a measuring cup or flat drinking glass to press the crust down until it feels firm to the touch. Place the pan into the center in the oven, on top of the stone or inverted baking sheet. Bake until lightly browned, around 10 minutes.
Remove the crust from the oven; it will still feel soft and damp. Immediately spread the chocolate chips evenly across the crust and return the pan to the oven for no more than 2 minutes, until the chocolate is softened. Using an offset spatula, spread the chocolate evenly across the crust. Refrigerate for at least 2 hours, up to 2 days.
Place an oven rack on the top shelf and set the broiler to high heat. Spread the marshmallows evenly over the chocolate layer. Place the pan under the broiler, then watch carefully. It takes about 2 minutes to brown the tops of the marshmallows perfectly. It takes about 4 minutes to destroy them.
To serve: While the s'mores pie is still hot from the oven, warm a long knife under hot water, make a cut and, wiping and warming the knife between subsequent cuts, portion the pie into serving pieces.
Make Ahead: The crust can be baked and the chocolate layer can be added over the crust 1-2 days in advance. Keep chilled in the refrigerator until just before adding the marshmallows. Recipe slightly adapted from Cathy Barrow's Pie Squared.