Stir together 4 cups milk, 1¼ cups sugar, and ¼ teaspoon baking soda in a heavy saucepan.
Bring to a boil, then reduce heat and simmer uncovered, stirring occasionally, until caramelized and thickened (about 1½ to 1¾ hours. After about an hour, stir more often as milk caramelizes, to avoid burning.)
