Place milk and sugar in a heavy bottomed pan, stir for 20 seconds with spatula
Place on cooktop and heat pan on low heat until sugar is dissolved, gently stirring here and there
Once sugar is dissolved, turn heat to medium and bring milk/sugar mixture to a simmer (do not stir from here on out)
Allow to simmer (not boil) for 40 minutes. Milk will thicken and darken a little. When done condensed milk should thickly coat a spoon and measure about two cups, but that measurement doesn't have to be exact. If it's not quite thick enough and you need to give it a little extra time, do! No harm will come from letting it simmer a little longer. Milk will also get thicker once it's placed in the refrigerator.
If a foam forms on top just skim off with a spoon
When ready, remove from heat and allow to cool
Once cool, if using vanilla, add it now and stir. Pour milk into a jar and place in the fridge. Allow condensed milk to get cold, (thickening even more) before using.