Prep the Pan: Preheat oven to 350°F (175°C). Line a 9x13-inch pan with parchment paper and lightly grease it.
Mix Brownie Batter: In a large bowl, whisk melted butter, granulated sugar, and brown sugar until smooth. Add eggs one at a time, then stir in vanilla.
Add Dry Ingredients: Sift in cocoa powder, hot cocoa mix, flour, and salt. Fold until combined. Pour batter into prepared pan and smooth top.
Bake Brownies: Bake for 25–30 minutes. Edges should be set but the center still soft. Test with a toothpick; a few moist crumbs are perfect.
Make Marshmallow Topping: Heat cream in a small saucepan over low heat. Remove from heat and stir in marshmallows and vanilla until smooth and fluffy.
Top the Brownies: Spread the topping over warm brownies. For toasted marshmallows, place under the broiler 1–2 minutes until golden brown.
Cool and Serve: Let cool for 10 minutes before slicing into squares. Enjoy with a cup of hot chocolate or milk.
