1Generously grease bottom and sides of 11x7-inch (2-quart) glass baking dish with 1 tablespoon butter; sprinkle with 1 tablespoon of the powdered sugar. In bowl of stand mixer, sprinkle unflavored gelatin over 1/2 cup cold water to soften; set aside.
2In 3-quart saucepan, heat granulated sugar, corn syrup, raspberry gelatin, salt and 1/2 cup water over medium heat, stirring constantly, until sugar and gelatin are dissolved. Heat to boiling; reduce heat to medium-low. 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. (While mixture is cooking, watch so it doesn’t boil over; reduce heat to low if necessary.)
3Slowly pour syrup into softened gelatin while beating on low speed. Increase speed to medium-high; beat 8 to 10 minutes or until mixture is light pink and has almost tripled in volume. Pour into dish; spread and smooth top with rubber spatula. Let stand uncovered at least 8 hours or overnight.
4Sprinkle large cutting board with about 1 tablespoon powdered sugar. Place remaining powdered sugar in small bowl. Loosen sides of marshmallow mixture from dish and gently lift in one piece onto cutting board. Spray sharp knife with cooking spray. Cut marshmallows into 1-inch squares (11 rows by 7 rows). Dip bottoms and sides of marshmallows into bowl of powdered sugar. Store tightly covered at room temperature up to 2 weeks.