Preheat oven to 325°F.
For the crust: In a medium bowl, mix together the graham cracker crumbs, melted butter and sugar. Press into an ungreased 8-inch or 9-inch pie dish. Set aside.
For the brownies: Melt the butter, sugar, cocoa, and salt in a large bowl in the microwave. Melt in 30 second increments and stir after each time until the butter is completely melted. The mixture will look gritty. Set aside for a few minutes as you get the other ingredients ready.
Mix in the vanilla extract. Add the eggs one at a time, mixing after each addition. Add in the flour, beating with a whisk until fully combined. Do not overmix. Spoon into the crust.
Bake in the preheated oven for 28 minutes. Remove from the oven and top with marshmallows. Return to the oven and continue to bake for an additional 3-5 minutes to melt the marshmallows. If the marshmallows aren’t toasted, turn on your broiler for 1 minute. Remove from the oven and allow to cool to room temperature before slicing. Run a very sharp knife under warm water when slicing; it helps slice neat pieces.
Pie tastes best at room temperature or reheated the day it is made. Any leftover slices should be heated for about 30 seconds in the microwave. Store pie at room temperature for up to 7 days. Do not freeze