Preheat oven to 350°F (163°C).
In a saucepan over medium heat, melt the butter. Once melted, remove from heat and whisk in the sugar, cocoa powder, and salt until combined and glossy.
Add vanilla. Let the mixture cool slightly (you don't want to scramble the eggs), then whisk in the eggs one at a time, beating well after each. The batter should be shiny and thick.
Stir in the flour just until no streaks remain—don’t overmix.
Fold in chocolate chips or chunks for extra richness.
Pour the batter into your graham cracker crust and spread evenly. Tap the pan lightly on the counter to settle it.
Bake for 25 minutes, remove the brownies and add your marshmallows on top of the brownies. Bake an additional 5-10 minutes, or until the center is just set and a toothpick comes out with a few moist crumbs (not wet batter).
You can also torch your marshmallows with a kitchen torch if you like them slightly burnt and toasted. Setting your oven to high broil for a few minutes with toast them as well if you don’t have a torch.
Let cool in the pan for at least 15–20 minutes before slicing. For clean cuts, cool completely and chill briefly before cutting.