Preheat the oven to 350 degrees. Line a 13- by 9-inch baking dish with foil. Spray the foil with cooking spray and set pan aside.

Combine the butter and unsweetened chocolate in a microwave-safe bowl and heat in 30-second intervals, stirring after each interval, until melted and smooth. (Or you can do this in the top of a double boiler set over barely simmering water.) Set the mixture aside to cool for 5 minutes.

Combine the sugar, eggs, vanilla, and espresso powder (if using) in a large mixing bowl. Stir vigorously with a rubber spatula until well combined. Stir in the chocolate mixture.

Add the flour and salt and mix until just combined, then fold in the chocolate chips and walnuts.

Scrape the batter into the prepared pan and bake for 32 to 38 minutes, until the brownies have just begun to pull away from the pan at the edges. Top with the marshmallows and return brownies to the oven for an additional 2 to 3 minutes, until the marshmallows have started melting together.

Cool completely before slicing and serving. Store uneaten brownies in an airtight container at room temperature for up to 2 days or in the freezer for up to 2 months.