In a microwave safe bowl, combine the chocolate, sweetened condensed milk, and butter.
Microwave for 30 seconds and then stir.
Continue microwaving the mixture using 15 second increments, stirring between each session, until the chocolate stirs smooth.
It should be thick and will pull away from bowl and spatula.
Add the vanilla extract and salt, and stir until combined.
Line a 9"x9" square cake pan or casserole dish with parchment paper.
Pour the fudge into your prepared pan and press it into place.
Cover with plastic wrap or a lid and refrigerate for two hours, or until firm.
Notes
Keep your kitchen equipment/utensils free of water when melting the chocolate. Getting water in the melting/melted chocolate will cause it to seize.
Making multiple flavors in a single batch: A full batch of the basic fudge weighs approximately one pound ten ounces. You can divide it accordingly to make multiple variations out of a single batch (I like to divide mine in half - so 13 ounces each - and make two different kinds).
Some dry ingredients, like cereals, will absorb some of the moisture from the fudge and will no longer be crispy.
Storage: Fudge can be stored in an air-tight container for up to 3 weeks in the fridge or 3 months in the freezer.