Preheat: Preheat your oven to 350°F, grease a 9x9-inch square pan, and set it aside.
Mix: In a large mixing bowl, combine the brownie mix, water, egg, and oil until smooth.
Bake: Pour the batter into the prepared pan and bake for about 30 minutes, or until a toothpick inserted in the center comes out clean.
Cool: Allow the brownies to cool completely to room temperature so the frosting won’t melt.
Warm Frosting: Warm the vanilla frosting in the microwave for about 30 seconds to make it easy to spread.
Color Frosting: Stir in 5–10 drops of green food coloring (or gel coloring) until the color is evenly combined.
Frost: Spread the green frosting over the cooled brownies for that signature Grinch look.
Decorate: Cut each brownie into nine squares and press a single red heart sprinkle into the top of each before serving. Enjoy!