Put 1 14 oz can sweetened condensed milk, 1 cup semi-sweet chocolate chips and 3 Tablespoons butter into a medium microwave safe bowl.
Microwave on high in 30-second intervals, stirring after each trip to the microwave, until everything has melted and the sauce is smooth.
Notes
This recipe makes about 2 cups of hot fudge, so you will likely have some left. Store it in an airtight container, I like using small glass jars, in the refrigerator for up to 2 weeks. Warm it up in the microwave or over low heat on the stove before using.
If you don't want to use a microwave, you can also make this hot fudge sauce in a small saucepan over low heat. Stir frequently until everything is melted and smooth.