Heat the oven to 350 degrees F. Stir the graham cracker crumbs, melted vegan butter, and 2 tablespoons granulated sugar together until evenly moistened.
Press the crust firmly into the bottom and sides of a 9-inch pie plate. Bake for 9 minutes, then cool completely.
Whisk the oat milk, coconut milk, 1/3 cup granulated sugar, cornstarch, and salt together in a saucepan. Cook over medium heat, whisking constantly, until thick and bubbling.
Remove the filling from the heat. Add the chopped chocolate and vanilla extract, wait for 1 minute, then whisk until glossy and smooth.
Pour the chocolate filling into the cooled crust. Cool for 30 minutes, then cover and refrigerate for at least 6 hours.
Arrange the mini marshmallows evenly over the chilled filling. Place the pie under a preheated broiler for 30 to 60 seconds, watching constantly, until the marshmallows are lightly toasted.
Sprinkle the pie with crumbled graham crackers, refrigerate for 15 minutes to reset the topping, and serve cold.
