This butterscotch sauce makes a great topping for ice cream or frozen yogurt. It can also be used as a sweet dip for sliced apples or pears.
In a small saucepan, combine sugar, water, and corn syrup; cook over medium heat, stirring constantly, until sugar has completely dissolved, about 3 minutes.
Add butter; stir until melted. Remove from heat, and stir in condensed milk. Serve warm.
Sauce will keep for several months in the refrigerator; cool completely, then transfer to an airtight container (label and date). Before serving, warm gently over low heat.