Preheat oven to 350°F. Line a 9×13-inch baking pan with parchment paper, leaving some overhang for easy removal.

In a large bowl, whisk together the melted butter and sugar until smooth.

Add the eggs and vanilla and whisk until well combined.

Add the cocoa powder, espresso powder, flour, and salt. Stir just until the dry ingredients are incorporated—don't overmix. Fold in the chocolate chips.

Pour the batter into the prepared pan and spread evenly.

Bake for 25–30 minutes, until the edges are set and the center still looks slightly fudgy. A toothpick should have a few moist crumbs attached.

Take the brownies out the oven and place the large marshmallows on top and broil for 2 minutes on the top rack. Watch carefully so it won't burn. Let this cool for 5 minutes before slicing. If you want your knife to slice smoothly, spray your knife with baking spray and cut your brownies.