In a medium saucepan, bring milk and sugar to a boil over medium heat.

Reduce heat and simmer for 2 hours or until the mixture has reduced by half. A skin may form on the top, remove as desired.

Stir in butter and vanilla and mix until smooth.

Pour into a clean glass jar and refrigerate at least 1 hour prior to use.

Store in refrigerator for up to 2 weeks.