In a medium saucepan over low heat, combine the chocolate and butter. Cook, stirring frequently until melted and smooth. Remove from heat and let cool slightly.
In a large bowl, combine the sugars, eggs, and vanilla and beat with a hand mixer until smooth.
Add the melted chocolate mixture and mix well.
Add the cocoa powder, espresso and salt and stir until combined.
Pour batter into prepared pan and bake until the edges of the brownies are mostly set, about 20 minutes.
Remove the brownies from the oven and pour marshmallows on top - spread evenly.
Return to oven and bake until the brownies are cooked through and the marshmallows are melted, about 10 - 12 minutes.
Let brownies cool.
To make fudge topping:
In a small saucepan over medium-low heat, combine chocolate chips, peanut or almond butter, and butter.
Cook, stirring often, until the chocolate and butter have melted and the mixture is smooth.
Remove from heat.
Spoon onto cooled brownies and spread evenly.
Sprinkle with course sea salt.
Notes
Let cool completely, then slice into squares.
You may also refrigerate brownies before cutting to make it easier to slice.