In 4-quart heavy nonstick saucepan, heat sweetened condensed milk, 1 tablespoon unsalted butter and coconut to boiling, stirring constantly with wooden spoon. Reduce heat to medium-low and continue to cook, stirring constantly, 10 to 15 minutes or until thick and shiny and starts to pull away from bottom and sides of pan. You will know mixture is ready when you tilt pan and mixture slides down easily. Transfer mixture to greased plate. Place in refrigerator or freezer until mixture is very cold.