Preheat the oven according to the directions on your brownie mix package. The brownie mix used for this recipe called for 350°F. Lightly coat a 9x9-inch baking pan with nonstick cooking spray, line it with parchment paper, and spray the parchment as well.
In a large mixing bowl, prepare the brownie batter according to the package instructions. For the mix used here, that included 1 egg, ¼ cup water, and ⅓ cup vegetable oil. Stir until well combined.
Pour the batter into the prepared pan and spread it into an even layer. Bake according to the package directions, about 25–30 minutes.
While the brownies are baking, combine the toasted coconut, cornstarch, and pink food coloring in a small bowl. Mix until the coconut is evenly coated and set aside.
As soon as the brownies come out of the oven, spread the marshmallow fluff evenly over the top. The heat from the brownies will soften the fluff, making it easier to spread.
Immediately sprinkle the pink coconut mixture over the marshmallow layer and gently press it into the fluff.
Refrigerate the brownies for at least 1 hour, or until fully chilled and set. You can also chill them overnight if needed.
Lift the brownies out of the pan using the parchment paper, slice into bars, and serve.