Pour the milk into a thoroughly washed and dried 250 mL canning jar, leaving a 1/4-inch of head-space at the top. (You’ll have a few drops of milk leftover; save it to sweeten a cup of tea or coffee with.) Put the screw band and snap-top lid on the jar and tightly seal. Set the jar in a slow cooker and cover it with hot water, letting the water rise 2-inches above it.
Cover and cook on the low setting for 8 hours, or until the milk has thickened and is a rich caramel colour. Turn off the slow cooker and let the dulce de leche sit in the hot water 1 hour. Remove the dulce de leche from the slow cooker, cool to temperature, and then store in the refrigerator overnight, or until needed. (The dulce de leche will have a smoother texture if you let it chill overnight.)
The dulce de leche will keep two weeks in the refrigerator, but you'll likely eat it up well before then!