In a heavy-bottomed pot, bring the milk and the sugar to a boil over medium heat.
Reduce the heat to low and simmer for about two hours until the volume is reduced by half. The mixture should be barely simmering and never bubbling at any point. Stir every 15 minutes or so to keep the milk from forming the "skin" on top. I forgot to stir and the milk burnt slightly and turn yellowish instead of remaining white.
After 2 hours, stir in butter.
Remove the pot from heat and let the milk cool. The mixture will thicken further after it has cooled.
When refrigerated, homemade sweetened condensed milk has thicker consistency than the commercial one. You can add a few drops of water and whisk it lightly to get a thinner mixture.
