Line a square 8 or 9 inch baking dish with aluminum foil. Make sure the foil goes up the sides of the pan because this fudge is tall! Set aside.
In a glass microwave safe bowl, combine the sweetened condensed milk and chocolate chips. Stir with a spatula until combined.
Microwave on full power for 1 minute and 30 seconds. Do not microwave longer, the mixture will melt and come together as you stir. Add in the salt and vanilla extract. Stir until the chocolate chips are melted and combined. Over microwaving can cause the chocolate to scorch and the fudge will be crumbly.
Transfer the fudge to the pan and spread into an even thickness. Allow the fudge to set on the counter for at least 6 hours, overnight is great too. You can loosely cover the top of the fudge with foil or plastic wrap.
Once the fudge is set, cut into squares. Store in an airtight container on the counter for up to 1-2 weeks.
To make chocolate walnut fudge, add 2 cups of chopped walnuts once the chocolate chips are melted.