Combine the whole milk and sugar in a heavy-bottom saucepan or a wider pan, like an enameled cast iron skillet. Stir to dissolve the sugar completely.
Place the pan over medium heat and bring the mixture to a gentle simmer. Reduce the heat to low and continue simmering. Stir occasionally to prevent the milk from scorching or forming a skin.
Allow the mixture to simmer gently for 30-40 minutes. The milk will gradually reduce and thicken. Be patient and avoid increasing the heat, which can cause the milk to scorch.
Remove it from the heat once the mixture has reduced to about half its original volume and has a creamy consistency. Remember, it will thicken further as it cools.
Let the condensed milk cool to room temperature. Pour it into an airtight container and refrigerate. It will keep in the fridge for up to a week.
Notes
Use the Right Cookware: A heavy-bottom saucepan is ideal to prevent scorching. Alternatively, I’ve found that using a wider pan, like an enameled cast iron skillet, helps the mixture reach the desired consistency more evenly.
Keep an Eye on It: While this recipe is simple, it’s important not to leave the saucepan unattended for a very long period of time.
Maintain a Low Simmer: Avoid boiling the mixture. A gentle simmer ensures the milk reduces slowly and evenly without boiling over.
Let It Thicken Naturally: The mixture will continue to thicken as it cools, especially after being refrigerated. If it’s still a bit runny when warm, don’t worry—it’ll set beautifully in the fridge.