- Generously grease bottom and sides of 11x7-inch (2-quart) glass baking dish with butter; dust with 1 tablespoon of the powdered sugar. In bowl of stand mixer, sprinkle gelatin over 1/2 cup cold water to soften; set aside.
- In 2-quart saucepan, heat granulated sugar, corn syrup, salt and 1/2 cup water over low heat, stirring constantly, until sugar is dissolved. Heat to boiling; cook without stirring about 30 minutes to 240°F on candy thermometer or until small amount of mixture dropped into cup of very cold water forms a ball that holds its shape but is pliable; remove from heat.
- Slowly pour syrup into softened gelatin while beating on low speed. Increase speed to high; beat 8 to 10 minutes or until mixture is white and has almost tripled in volume. Add peppermint extract; beat on high speed 1 minute. Pour into baking dish, patting lightly with wet hands.
- Drop food color randomly onto top of marshmallow mixture. Pull table knife through food color to create swirl pattern over top. Let stand uncovered at least 8 hours or overnight.
- Dust cutting board with about 1 tablespoon powdered sugar. Place remaining powdered sugar in small bowl. To remove marshmallow mixture, loosen sides from dish and gently lift in one piece onto cutting board.
- Using sharp knife greased with butter, cut into 1-inch squares (11 rows by 7 rows). Dust bottom and sides of each marshmallow by dipping into bowl of powdered sugar. Store in airtight container at room temperature up to 3 weeks.