Stir together graham crumbs and butter until all crumbs are evenly coated. Press into and up the sides of a 9-inch pie plate. Set aside.
For the pie filling:
Preheat oven to 350°F.
Combine sugar and cocoa powder in a large bowl; add eggs and buttermilk, stirring until blended. Stir in vanilla, melted butter, and melted chocolate. Pour into the prepared graham cracker crust.
Bake for 45 minutes, tenting with foil after about 20 minutes of baking. Let pie cool completely.
For the marshmallow topping:
Fill a medium to large pot with 1-inch water and bring to a simmer. In a heat-safe bowl, or the bowl of a stand mixer, whisk together egg whites, sugar, cream of tartar, and salt.
Place the bowl over the pot, making sure the bowl does not touch the simmering water. Cook egg white mixture, whisking constantly, until sugar dissolves and mixture reaches 120°F on a candy thermometer.
Remove from heat and whip mixture on high speed until stiff, glossy peaks form.
Spread topping over the cooled pie, creating large swoops with a rubber spatula or the back of a spoon.
In your oven, place the rack in the middle; preheat broiler. Place pie under the broiler and let it toast the top of the marshmallow topping for 30 seconds to 1 minute. Do not walk away from your oven! (I cracked the oven door and watched it the entire time it was toasting.)
If you have a kitchen torch, feel free to toast the topping that way!
Once toasted, allow the pie to cool before serving; pie is best served warm. Store covered in the refrigerator.