Line an 8x8-inch square baking pan with parchment paper, leaving some overhang on the sides for easy removal.
In a medium saucepan over low heat, combine the chocolate chips, sweetened condensed milk, and butter.
Stir constantly until the chocolate is completely melted and the mixture is smooth, about 5-7 minutes.
Remove from heat and stir in the vanilla extract and salt. If using nuts, fold them in now.
Pour the fudge mixture into the prepared pan and spread evenly with a spatula.
Refrigerate for at least 2 hours or until completely firm.
Using the parchment paper, lift the fudge out of the pan and cut into 1-inch squares. Store in an airtight container in the refrigerator for up to 2 weeks.