When you forget to buy it, you can make sweetened condensed milk at home. I use it all the time, particularly in Three-Ingredient No-Churn Vanilla Ice Cream and Single-Serving Key Lime Pie Cheesecake. It keeps in the fridge for months.
Makes 1½ cups (425 milliliters)
1 In a heavy-bottomed medium saucepan, combine the milk and sugar and cook over medium heat, stirring, until the sugar has dissolved. Reduce the heat to low and bring the mixture to a simmer. Do not stir the mixture once it starts to simmer, or it may crystallize.
2 Gently simmer for 45 to 50 minutes, until the milk has become a rich, creamy color, and has reduced by half and thickened slightly. You may notice some foam forming on the surface as it simmers; gently skim it off with a spoon. (If there is undissolved sugar hanging around the rim of your pot, don’t stir it into your condensed milk; this can also cause crystallization.) Take care not to reduce the milk too much, or the fat will separate.
3 Remove from the heat and pour the milk into a jar to cool completely. Seal the jar and refrigerate for a few hours before using (the milk will thicken up a lot as it chills).
4 Label the jar with the date and store in the fridge for up to 6 months.
Notes
You can use whole, low-fat, or even nondairy milk to make condensed milk.
A sugar substitute like stevia will not work in this recipe, as it does not caramelize.