In a large heavy-bottomed saucepan, mix together 2 cups of milk and ⅔ to ¾ cup of sugar. Add a pinch of salt.
Place the saucepan over medium-low heat, stirring continuously until the sugar completely dissolves into the milk.
Increase the heat to medium and bring the mixture to a gentle simmer, not a full boil. Stir occasionally to prevent sticking and ensure even reduction.
Let the mixture simmer and reduce by about half, which may take 30 to 45 minutes. As it thickens, stir more frequently to prevent burning.
The mixture is ready when it coats the back of a spoon but is still pourable. Remember, it will thicken as it cools.
Remove from heat and, if using, stir in 1 teaspoon of vanilla extract for added flavor.
Allow the sweetened condensed milk to cool to room temperature, stirring occasionally to prevent a skin from forming.
Transfer the cooled mixture to a clean, airtight container, such as a mason jar, and refrigerate. It can be stored for several weeks.