Preheat your oven to 350°F (175°C). Grease a baking dish or line it with parchment paper.

Melt 10 tablespoons of unsalted butter and 2/3 cup of dark chocolate chips together in a saucepan over low heat. Stir until smooth.

Remove from heat and whisk in dark brown sugar and granulated sugar until well combined.

Add the eggs, egg yolk, and vanilla extract, mixing well.

In a separate bowl, sift together the flour, cocoa powder, espresso powder (if using), and fine sea salt. Gradually fold this into the chocolate mixture until just combined.

Pour the brownie batter into the prepared dish, smoothing it out evenly. Sprinkle with 1/2 cup of chopped dark chocolate.

In a small bowl, combine 1/3 cup water with the gelatin powder and let it sit for about 5 minutes until it blooms.

In a saucepan, combine 1 1/8 cups granulated sugar, 1/4 cup + 1 tablespoon agave, 1/3 cup water, and fine sea salt. Cook over medium heat until the mixture reaches 240°F (115°C) on a candy thermometer.

Remove from heat and add the bloomed gelatin and vanilla bean paste, whisking until dissolved.

Using an electric mixer, whip the marshmallow mixture on high until thick and fluffy, about 5-7 minutes.

Spread the marshmallow over the brownie layer evenly.

Bake for 10-12 minutes until the marshmallow is lightly toasted. Let it cool before slicing into squares.