Preheat oven to 350°F (177°C).

Mix the graham cracker crumbs and granulated sugar together in a medium bowl, and then stir in the melted butter. The mixture will be thick, coarse, and sandy. Try to smash/break up any large chunks. Pour the mixture into an ungreased 9-inch pie dish. With medium pressure using your hand, pat the crumbs down into the bottom and up the sides to make a compact crust. Do not pack down with heavy force because that makes the crust too hard. Simply pat down until the mixture is no longer crumby/crumbly. Tips: You can use a small flat-bottomed measuring cup to help press down the bottom crust and smooth out the surface, but do not pack down too hard. And run a spoon around the bottom “corner” where the edge and bottom meet to help make a rounded crust⁠—this helps prevent the crust from falling apart. For more shaping technique tips, see the graham cracker crust recipe page.

Slice the butter into Tablespoon-size pieces and melt in the microwave in a large heatproof bowl or in a small saucepan on the stove. (Transfer melted butter to a large bowl if using the stove.) Whisk the oil and granulated sugar into the melted butter. Let it sit for 3–5 minutes to slightly cool. Whisk in 2 whole eggs, 1 egg yolk, and vanilla extract until combined. Add the cocoa powder, flour, and salt and whisk until combined. Batter will be thick. Fold in the chocolate chips. Batter will continue to thicken the longer it sits, so immediately spoon and spread evenly into crust.

Bake for 30 minutes. Remove from the oven and arrange marshmallows on top, pressing each very lightly down into the filling to help them stick. Return pie to the oven and continue to bake for an additional 3 minutes to toast the marshmallows. If the marshmallows aren’t toasted to your liking after this time, turn on the broiler for 30–60 seconds and remove the pie as soon as the marshmallows look toasted. Do not walk away from the oven because toasting can happen fast and the marshmallows and crust can quickly burn.

Remove pie from the oven and cool for at least 45 minutes on a wire rack. For best presentation and neatest slices, allow to cool for closer to 2 hours or cool completely. For extra neat slices, run a very sharp knife under hot water and wipe dry between slices.

Cover and store leftover pie at room temperature for up to 3 days or in the refrigerator for up to 1 week. You can reheat slices for a few seconds in the microwave if desired.