Gather your ingredients: Ensure you have 1 cup of whole milk, 1 cup of granulated sugar, 1 tablespoon of butter (optional), and 1 teaspoon of vanilla extract (optional).
Mix milk and sugar: In a medium saucepan, combine the 1 cup of whole milk and 1 cup of granulated sugar. Turn the heat to medium and stir the mixture gently until all the sugar has dissolved. This step should take just a few minutes.
Simmer the mixture: Reduce the heat to low and allow the mixture to simmer. Keep stirring occasionally to prevent the milk from scorching on the bottom of the pan. Continue this simmering process for 30-40 minutes. The goal is to reduce the liquid volume by about half and achieve a thicker consistency.
Check for consistency: After about 30-40 minutes, you should notice that the mixture has thickened significantly. The best way to check is to dip a spoon into the mixture; it should coat the back of the spoon when it’s ready. You’re looking for a pourable but thicker consistency.
Add flavorings: If you wish for a richer flavor, stir in 1 tablespoon of butter and 1 teaspoon of vanilla extract at this point. Mix well until the butter has melted and blended into the mixture.
Cool and store: Once your condensed milk is ready, let it cool to room temperature before transferring it to an airtight container. Store in the refrigerator where it can last up to 2 weeks.