Preheat the oven to 350°F (177ºC). Lightly spray a 9-inch pie dish with nonstick cooking spray and set aside.
In a medium skillet, toast the graham cracker crumbs (1 1/2 cups) over medium heat for 3–4 minutes, stirring often, just until fragrant and lightly golden. Transfer to a bowl and add brown sugar (1/4 cup), and salt (1/4 teaspoon). Pour in the melted butter (6 tablespoons) and stir until evenly moistened.
Press the mixture firmly into the bottom and up the sides of the prepared dish using your fingers or the bottom of a flat measuring cup.
Bake for 6–7 minutes, then remove and cool slightly. Reduce the oven temperature to 325°F (163ºC).
Make the Chocolate Filling
In a small saucepan over medium-low heat, heat the heavy cream (3/4 cup) until it just begins to simmer — look for tiny bubbles around the edges. Remove from heat and add semi-sweet chocolate chips (6 ounces), milk chocolate chips (1/2 cup), and butter (2 tablespoons). Let sit for 2 minutes, then whisk until smooth. Stir in the vanilla (1 teaspoon), espresso powder (1 teaspoon), corn syrup (1 tablespoon), and salt (1/4 teaspoon).Let the mixture cool for 5–10 minutes, stirring occasionally.
Meanwhile, in a medium bowl, whisk the cornstarch (1 tablespoon) with one egg until smooth. Add the second egg and whisk again until fully combined.
Temper the eggs: slowly whisk 2–3 tablespoons of the warm chocolate mixture into the eggs to gently raise their temperature. Then whisk the tempered eggs back into the chocolate mixture until completely smooth and glossy.Pour the filling into the cooled crust.
Bake the Pie
Bake at 325°F (163ºC) for 20–25 minutes, or until the edges are set and the center jiggles slightly when nudged. Remove and cool on a wire rack for 30–60 minutes, then refrigerate for at least 4 hours (or overnight) to fully set.
Toast the Marshmallow Topping
Once the filling is chilled and firm, top the pie evenly with mini marshmallows (2 1/2 cups). Nestle in Hershey’s chocolate squares and sprinkle with graham cracker crumbs (1 tablespoon).
For a golden, gooey finish, toast the topping under the broiler for 30–90 seconds (watch it closely!) or use a kitchen torch for more control.
Cool for 5–10 minutes, then sprinkle lightly with flaky sea salt for a sweet-salty pop.Slice with a hot, dry knife, wiping the blade clean between cuts for neat slices.
Notes
Chill overnight if possible: The texture becomes even silkier after a full chill.
Use a hot, dry knife: Run it under hot water, wipe clean, and slice — your layers will stay gorgeous.
Add a pinch of flaky sea salt on top: It’s a tiny touch that makes the chocolate flavor sing.
Make it your own: Stir 2 tablespoons of Irish cream liqueur into the chocolate for a grown-up twist.