First generously grease bottom and sides of 11x7 inch glass baking dish with butter. Then 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.
Next in 2quart saucepan heat 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.
Next slowly pour syrup into softened gelatin while beating on low speed. Then increase speed to high & beat 8 to 10 minutes or until mixture is white & has almost tripled in size. Then add in the peppermint extract & beat on high speed 1 for about minute. Then pour into baking dish & patting lightly with wet hands. Drop food color randomly onto top of marshmallow mixture. Use a table knife through food color to create swirl pattern over top. Let stand uncovered at least 4 to 8 hours or overnight.
Then dust a cutting board with about 1 tablespoon powdered sugar. Place remaining powdered sugar in medium bowl. Then remove marshmallow mixture by loosen sides from dish & gently lift in one piece onto cutting board. Use sharp knife greased with butter & cut into 1-inch squares. 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.