Line a 9×13-inch pan with foil. Lightly coat the pan with nonstick cooking spray.
In a small bowl, sift together the powdered sugar and cornstarch. Sift 2 tablespoons of the powdered sugar mixture into the pan and tilt to coat all sides.
Pour the water into the bowl of a stand mixer. Sprinkle evenly with gelatin on top. Let it bloom while preparing the next step.
In a medium heavy-bottomed saucepan, heat sugar, corn syrup, peppermint pieces, and salt over medium heat. Cook until completely dissolved, about 7 minutes, stirring constantly.
Pour the hot mixture into the bowl with the gelatin mixture. Beat on high speed with the whisk attachment until light and fluffy, about 10-15 minutes.
Working quickly while the marshmallow is still soft, spread evenly into the prepared pan. Smooth the top with a spatula coated in cooking spray. If using, add scattered dots of food coloring on top of the mixture. Swirl a knife or toothpick through the drops to create a marbled pattern on top. Let the marshmallow set at room temperature for 2 hours, loosely covered with foil or plastic wrap.
After the marshmallow has set completely, invert the pan onto a cutting board. Remove the foil and gently turn the marshmallow over face up. With a cookie cutter or knife lightly coated in cooking spray, cut to your desired shapes. Dip the cut sides in the remaining sugar/cornstarch mixture. Pat off any excess.
Enjoy immediately, or store at room temperature in an airtight container.