Preheat oven to 350F (325F convection bake). Line a 9”x 9” baking pan with parchment paper. Cover surface of lined pan with one layer of graham cracker cookies. Set aside.
In a large mixing bowl, mix melted butter with sugar and cocoa powder until a wet, dark brown, mixture forms.
Add vanilla, and eggs to your mixture. Mix until a smooth and silky chocolate mixture forms (1-2 min).
Add flour, espresso powder, and salt to batter. Mix ingredients together with a whisk or spatula until combined.
Fold your milk chocolate chips/pieces into the batter.
Add brownie batter to your prepared pan lined with graham crackers. Spread flat.
Bake in a preheated oven for 30-35 minutes, or until edges are slightly raised and middle is no longer jiggly. Remove from oven.
Increase oven temperature to 425F (Broil). Cut marshmallows in half (if not already done).
Add your large marshmallow halves to the surface of the brownie pan (note: I like to adhere the sticky part of the marshmallow half to the brownie to prevent it from sliding). Continue to add marshmallows, side by side, until brownie is completely covered.
Return brownies to the oven and watch carefully as they toast over 30 seconds to 2 min - depending on your oven. (note: this steps happens fast!)
Let brownies cool completely in baking pan.
Cut into squares and enjoy!