Preheat oven to 350°F and lightly grease a 9×13-inch pan. Line with parchment paper for easy removal.

In a medium bowl, whisk 2 large eggs, ½ cup vegetable oil, and ½ cup water until glossy and smooth.

Stir in the 18.3-ounce brownie mix until no dry streaks remain.

Spread batter evenly in the prepared pan and bake 20–22 minutes or until edges are set and a toothpick inserted comes out with a few moist crumbs.

Allow brownies to cool in the pan for 10 minutes on a wire rack before frosting.

In a large bowl, beat ½ cup softened unsalted butter on medium speed for 1 minute until creamy.

Add 7 ounces marshmallow cream, 1 teaspoon vanilla extract, and a pinch of salt. Beat until smooth and combined.

Gradually add 2 cups sifted powdered sugar, beating after each addition. If frosting is too stiff, add up to 2 tablespoons milk.

Spread frosting over the warm brownies and top with sprinkles.

Chill in the refrigerator for 10 minutes to set the frosting, then cut into squares.