Combine milk and sweetener in a medium saucepan on medium heat.
Bring to a boil, whisking continually.
Reduce and slowly cook on the lowest temperature until you have approximately 1⅓ cup. This may take two hours or more. Stir often, but not continually.
When milk is reduced by half (resulting in approximately 1⅓ cup), remove from heat and whisk in butter and vanilla extract.
Pour into a clean glass jar and allow to cool. Store in the refrigerator or use immediately. Mixture will thicken more after being refrigerated, so I like to chill then use later.
This was thinner than sugar-filled, store-bought sweetened condensed milk (though it does thicken in the refrigerator more). I’ve toyed with the idea of sprinkling a ½ teaspoon gluccomannan over it and whisking quickly and/or loosening a half a bar of cream cheese (or more) in microwave or stove top and whisking it into the mixture.
Recipe by Donna Reish at https://donnareish.com/3-sugar-free-sweetened-condensed-milk-recipes/