In a large microwave-safe bowl, combine the chocolate chips, sweetened condensed milk, and butter.
Microwave in 30-second intervals, stirring well each time, until completely melted and smooth. This usually takes about 1½ to 2 minutes total.
Pour the mixture into a greased or parchment-lined baking dish (8x8 or 9x9 inch) or divide into small tins for gifting.
Let the fudge set at room temperature for a few hours, or chill in the refrigerator or freezer to set more quickly.
Cut into squares and enjoy! Store in an airtight container at room temperature or in the fridge.