Line an 8 x 8-inch pan with parchment paper and set aside.
In a microwave-safe bowl, combine your condensed milk with chocolate. Microwave in 20-second spurts until the chocolate is mostly melted. Add the vanilla extract and whisk together until smooth.
Transfer the mixture into the lined pan and refrigerate for at least an hour, to firm up. Once firm, slice into pieces.
TO STORE: Fudge is best stored in the refrigerator, covered. It will keep well for at least a month, if not longer. TO FREEZE: Place leftover fudge in a shallow container and store it in the freezer for up to 6 months.