Heat oven to 350°F. Lightly butter a 9-inch pie dish.
Stir graham crumbs, sugar, and melted butter until evenly moistened.
Press mixture firmly across the bottom and up the sides of the pie dish.
Bake crust 8–10 minutes until fragrant. Cool on a rack while you make the filling.
Warm cream in a saucepan over medium heat until steaming with small bubbles at the edges.
Pour hot cream over chocolate in a bowl. Let stand 2 minutes, then whisk smooth. Blend in vanilla and salt.
Spread chocolate filling into the cooled crust. Chill until set, about 2 hours or until firm to the touch.
Top with marshmallows. Broil on the middle rack 1–2 minutes, rotating the pie, until toasted. Watch closely.
Let stand 5–10 minutes before slicing with a warm knife for clean cuts.