Make eyes and a mouth of each marshmallow using tube black frosting, set aside. You can also use an edible pen marker.
Place 2 cups marshmallow fluff in a large microwave-safe bowl. Heat until it starts to puff, usually around 30 seconds, remove and stir.
Immediately spoon onto each of the baked and cooled 12 brownies. If using sprinkles, sprinkle on while still tacky. Top with the marshmallow face so it sticks.
Repeat with the other half of the brownies. Marshmallow fluff cools fast and won't spread and drip, so working in 2-3 batches works best.
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Notes
Make-Ahead: You can bake the brownies ahead of time and decorate when you’re ready.Refrigerator: These brownies will last for up a week in the fridge when stored in an airtight container or resealable plastic bag. You can also wrap them in plastic wrap, but it could affect the appearance of the decorated marshmallow.Freezer: While you can freeze brownies for up to 3 months, the decorative topping may not retain its appearance and texture after thawing.