Whisk together the milk and maple syrup in a crockpot.
Turn it on high and leave the lid off so the liquid can evaporate and reduce.
Occasionally give it a good whisking. Whenever you remember.
When it has reduced to about 28 ounces and thickened up, you're all done. I eyeballed it and only had to weight it twice before I got the right amount.
Whisk in the butter and vanilla.
Refrigerate until you want use it.
Recipe by Reformation Acres at https://www.reformationacres.com/2015/01/how-to-make-sweetened-condensed-milk.html