Over a double boiler, melt Butter (1/2 cup) and Semi-Sweet Chocolate Chips (8 ounce). Add Granulated Sugar (3/4 cup) and Brown Sugar (1/4 cup). Remove from heat and set aside to cool completely.
In a mixing jug, add Egg (2) and Vanilla Extract (1 teaspoon). Whisk to combine. Add egg mixture to the chocolate mixture and whisk again until smooth.
In a mixing bowl, sift together All-Purpose Flour (3/4 cup), Unsweetened Cocoa Powder (2 tablespoon), and Salt (1/4 teaspoon). Stir to combine.
Fold the dry ingredients into the wet ingredients.
Transfer mixture to a 9-inch square pan. Bake at 350 degrees F (180 degrees C) for 35 minutes. Set aside to cool.
In a heavy-bottomed saucepan, add Mini Marshmallows (1 cup), Milk (1/4 cup), Semi-Sweet Chocolate Chips (4 ounce), Vanilla Extract (1/2 teaspoon). Melt together until mixture is smooth. Set aside to cool.
In the bowl of a stand mixer, add Heavy Cream (1 cup). Mix until cream forms stiff peaks.
Add cooled chocolate and marshmallow mixture to the cream. Fold together gently.
Spread chocolate mousse over cooled brownies. Place in fridge to cool for 30 minutes.
Drizzle Chocolate (to taste) over top.