Heat the oven according to the brownie-mix package directions, usually to 350°F. Grease or line a 9-by-13-inch baking pan with parchment paper.

Stir together the brownie mix, eggs, vegetable oil, and water until just combined. Avoid overmixing so the baked brownies remain tender and fudgy.

Spread the batter evenly in the prepared pan. Bake according to the package directions, usually for 22–28 minutes, until the center is set and a toothpick shows a few moist crumbs rather than wet batter.

Cool the brownies completely in the pan. For cleaner cuts and firmer frosting, chill the cooled brownies for 20–30 minutes before slicing.

Cut the brownies into 16–24 squares or rectangles. Spoon or pipe vanilla frosting onto each piece in a tall, rounded mound to resemble a ghost.

Press two mini chocolate chips into each frosting ghost for the eyes. Add a third mini chocolate chip or a small dot of chocolate frosting for a mouth, if desired.

Serve the brownies immediately or refrigerate them until the frosting firms. Keep decorated brownies covered so the frosting stays fresh.