Add butter, chocolate chips, milk, and vanilla extract to a large bowl that is microwave-safe.
Microwave fudge mixture for one minute at a time, and then stir. Repeat for a total of three minutes.
Mix in sifted powdered sugar into the chocolate mixture while it is still hot.
Pour fudge mixture into a small square pan lined with greased parchment paper or aluminum foil to prevent sticking.
Place fudge in the refrigerator to firm up for at least six hours. Then slice and serve.
Notes
I think the microwave is the best option because of its simplicity, however, you can also melt the butter, chocolate, and milk mixture in a saucepan set over low heat on the stovetop if you don't wish to use a microwave.