Mix sugar and milk together in a heavy-bottomed saucepan.
Stirring often, bring to a low simmer over medium-low heat. Make sure it doesn’t boil!
As soon as steam starts lifting off the milk, lower the heat even further, and when the sugar is entirely dissolved, put the heat as low as it can go.
The goal is to reduce the quantity in the pot (which is now about 1 ¾ cups) by approximately half.
It takes about 2 hours at very low heat to reduce to one cup of liquid.
Once reduced to your satisfaction, whisk in the butter and vanilla.
