Preheat oven to 400°F and line a baking sheet with parchment paper.
Roll out pie dough to about a 12-inch circle and place on the prepared baking sheet.
Sprinkle ½ cup mini marshmallows in the center of the dough, leaving about a 1½-inch border. Top with all chocolate chips, then sprinkle crushed graham crackers evenly over the chocolate.
Fold the edges of the dough up and over the filling, pleating as needed.
Whisk together egg and heavy cream. Brush the exposed edges of the dough with the egg wash.
Bake for 25 minutes, until the crust is golden and the chocolate is melted.
Remove from the oven, scatter remaining marshmallows over the top, and broil on high for 1–2 minutes, watching closely, until marshmallows are toasted to your liking.
Remove from oven and sprinkle lightly with flaky sea salt.
Serve warm, sliced like a pie or scooped with graham crackers or cookies.