Step 1

Put sugar and milk in medium saucepan. Heat over medium heat until it just starts to bubble, then reduce the heat to medium low. The mixture will foam up, so be sure to have room for the mixture to grow in the pan.

Step 2

Simmer over medium-low heat until the volume is reduced to 1 cup, this should take between 30 and 45 minutes.

Step 3

Remove from the heat and stir in the butter and vanilla.

Step 4

Store in a covered container in the refrigerator for up to one week. Stir before using.