Before you make the pie, leave 4 oz of unsalted butter, to be used for the filling, out on the counter to soften at room temperature.
To prepare the crust, put flour, sugar and salt in a small bowl and mix thoroughly with a whisk. Then transfer to a food processor fitted with the dough blade.
Cube the cold butter and add to the flour mixture. Pulse several times until butter is about pea size and mixture resembles coarse crumbs.
Drizzle cold water over the dough mixture, and pulse until the dough starts to pull together. Be mindful not to overmix, you still want chunks of butter in the dough to create a flaky crust.
Turn the dough out on the counter or cutting board and shape into a disk. Wrap in plastic and rest in the refrigerator for an hour.
Take pie dough out of the fridge and roll into a circle about 2” larger than your pie dish. Use extra flour to keep dough from sticking to the counter and roller.
Carefully place the dough over the pie dish, lightly press the dough into the bottom and side of the pie dish, let the excess dough hang over the side.
Trim some of the excess dough if necessary to create an even edge, but leave enough to fold under and flute the edge.
Cover the prepared pie dough with plastic wrap and freeze for 10 - 15 minutes.
Preheat the oven to 425°F.
Take the frozen pie crust out of the freezer and poke holes with a fork on the bottom and side.
Line the frozen pie crust with parchment paper, large enough to cover the crust completely.
Add pie weights or dry bean on top of the parchment paper, enough to fill to the top.
Par-bake pie crust for 20 minutes.
Remove pie weights or bean, and bake for another 5 minutes.
Remove from the oven and let cool on a wire rack.
Reduce oven temperature to 350°F.
To prepare the filling, beat soften butter with sugar and salt until completely combined.
Whisk together egg, cream and vanilla in a small bowl.
Add egg mixture to butter mixture and beat to combine. Stop and scrape the side a few times to make sure all the butter is completely incorporated.
Break up the graham crackers and spread half of the pieces to cover the bottom of the pie crust.
Add the chocolate square for the second layer.
Sprinkle the marshmallows on top of the chocolate layer.
Add the rest of the graham cracker pieces on top of the marshmallows.
Pour the prepared custard mixture over the top, shake it around to make sure it fills up all the voids.
Cover the edge of the crust with foil.
Bake pie for 45 minutes.
Remove foil and bake for another 5 minutes.
Remove from oven and let cool complete on a wire rack.