Preheat the oven to 350 and set a pie dish to the side.
Make the graham cracker crust. If using graham crackers that are not pre-crushed, crush them into a fine texture using a food processor or add them to a plastic bag and smash them with a rolling pin. Mix the graham cracker crumbs with the melted butter and sugar. The texture of the mixture should be sandy. Add the crushed graham crackers to the pie dish and press it down to cover the bottom and the sides of the dish. Set to the side.
Make the brownie mixture. Melt the butter and set it to the side to cool. In a large bowl, combine the oil and sugars with a whisk. Then, add in the cooled melted butter, followed by the eggs and vanilla. Mix in the cocoa powder, flour, salt, and espresso powder. Fold in the chocolate chips.
Pour the brownie mixture on top of the graham cracker crust and place the pie in the oven to bake. Bake the pie for about 25-30 minutes until the brownies start to crack on top.
When the brownies are done, take the pie out of the oven and add the marshmallow, making sure to cover the surface layer of the pie. Turn the oven to broil and toast the marshmallows until golden brown, about 30-60 seconds.
For a clean slice of the pie, let the s'mores brownie pie sit at room temperature for about 30-40 minutes.