Mix together the graham cracker crumbs, sugar, salt, and the melted butter until well-combined.
Line an 8 x 8-inch baking pan with parchment paper and fill with the graham cracker crumbs. Pour the crumbs in the bottom of the pan and pack the crumbs down as much as possible. Preheat the oven to 350F.
Partially melt the butter, chocolate, and sugar halfway on medium-low heat. Remove from heat and mix well until the butter and chocolate fully melts. Beat the eggs into the chocolate very well. Stir in the vanilla.
Mix in the cocoa powder, flour, and salt together in the chocolate mixture until just combined.
Pour the batter over the graham cracker crust and spread to the edges.
Bake for 35 - 40 minutes or until a toothpick comes out with thick, moist crumbs sticking to it. Place marshmallows over the top of the brownies and bake for an additional 1 – 2 minutes. Remove and optionally torch the tops of the marshmallow. You can also try to use the broiler in the oven and keep a careful eye on the toasting.
Cool for at least 10 minutes before lifting out of the pan and cooling on a wire rack completely before slicing.
Notes
Use a mix of small and large marshmallows for a fun and beautiful textural difference on top of the brownies. You can also use homemade marshmallows.