Preheat the oven and line a square baking pan with parchment paper before covering the bottom with a single layer of graham cracker sheets.

Melt the butter and the first portion of chocolate chips together until the mixture is completely smooth and glossy.

Whisk the sugars, eggs, egg yolk, and vanilla in a separate bowl until the mixture is pale and well combined.

Slowly stream the melted chocolate into the egg mixture while whisking constantly to keep the eggs from cooking.

Sift in the flour, cocoa powder, espresso powder, and salt, then gently fold the dry ingredients into the batter until just combined.

Pour the finished batter over the crackers and bake until a toothpick comes out with only a few moist crumbs.

Bloom the gelatin in water while heating the sugar, agave, and additional water in a saucepan until it reaches a boil and thickens.

Slowly pour the hot syrup into the gelatin while whipping at high speed until the mixture becomes thick, white, and fluffy.

Spread a thin layer of melted chopped chocolate over the cooled brownies before topping with the fresh marshmallow mixture.

Allow the layers to set for at least four hours at room temperature to ensure the marshmallow is stable enough to slice.

Toast the top of the marshmallow layer with a kitchen torch or briefly under a broiler for a golden, charred finish.