Add sugar and butter to a small saucepan over medium heat. As the butter melts, stir to combine with the sugar. Meanwhile, microwave the milk to warm it.

Caramelize the sugar: cook to a deep amber. Be careful not to let it burn! You can also caramelize less if you want a less intense caramel flavor. When sugar is ready, remove from heat. Add milk in a steady stream as you stir. The sugar will seize up and there will be sizzling, but keep stirring.

Place back on stovetop over low heat. Make sure to keep the heat sufficiently low so that you don't boil the milk. Continue stirring until all of the caramelized sugar is melted into the milk.

Store the left overs in the fridge.