1. Lightly oil a 9-inch-square pan. (A smaller pan will make the fudge thicker and more difficult to cut.) Place milk in a heavy 12-quart saucepan. Stir in sugar; add butter. Over medium heat, stir constantly, until sugar dissolves and butter melts. Increase heat to medium high, bring to a boil, and cover with lid. Let boil for 2 minutes, and uncover. Do not stir; clip a candy thermometer on side of pan, and let mixture boil, over medium-low heat, until it reaches 240 degrees (the soft-ball stage), 15 to 25 minutes. Meanwhile, prepare an ice water bath.

2. Immediately remove pan from heat, and dip the base of the pan in the ice water for 5 seconds. Let the fudge sit at room temperature without stirring, with the thermometer attached, until the thermometer reads 122 degrees and mixture is lukewarm, about 40 minutes. If using food coloring, add 1 drop. Stir the fudge with a wooden spoon until it changes from glossy to opaque and is thick and creamy, 2 to 5 minutes. Quickly pour into prepared pan. Use piece of plastic wrap to smooth fudge into the pan, creating an even surface. Let cool completely, 2 to 3 hours, and cut with 1/2- to 1 1/2-inch heart cookie cutters. (Oil the cutters with the canola oil inside and out before using.) Fudge will keep, well wrapped in plastic or in an airtight container, up to 1 week.