Clip on a candy thermometer; raise heat to high. Cook syrup without stirring until it reaches 244 degrees (firm-ball stage). Immediately remove pan from heat. With mixer on low speed, slowly and carefully pour syrup into the softened gelatin. Increase speed to high; beat until mixture is very thick and white and has almost tripled in volume, about 15 minutes. Add vanilla; beat to incorporate. Generously dust an 8 x 12-inch glass baking pan with confectioners' sugar. Pour marshmallow mixture into pan. Dust with confectioners' sugar; let stand overnight, uncovered, to dry out. Cut marshmallows with sprayed and dusted cookie cutters for fun shapes. Lightly dust remaining sides with powdered sugar. Store in an airtight container up to 2 weeks.