Line an 8x8-inch pan with foil and spray with nonstick spray (or rub lightly with butter).
In a large bowl, melt the butter in the microwave (about 40 seconds).
Add the chocolate chips and the sweetened condensed milk to the bowl and stir.
Microwave again for 30 seconds. Stir again. Repeat 5 more times- microwaving for 30 seconds each time, then stirring after each burst in the microwave.
Pour the fudge into the prepared pan and smooth to even in the pan. Refrigerate until firm. Cut into pieces.
Notes
Use peanut butter chips to make peanut butter fudge!