Melt butter in a small saucepan, over medium heat.

Heat evaporated milk in a large microwave safe dish, for 60 seconds in the microwave.

Quickly whisk sugar into warm milk, until sugar is dissolved.

Add milk mixture to melted butter and whisk continuously over medium-high heat. Bring to a boil (sauce will slightly thicken) and remove from heat. Serve right away and refrigerate leftovers.