Start by lining your baking pan with parchment paper or foil so the fudge doesn’t stick.
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 smooth. This gentle heat keeps the chocolate from seizing.
Once smooth, remove the pan from the heat and stir in the chopped walnuts if you’re using them.
Pour the fudge mixture into the prepared pan and spread it evenly. Let it cool at room temperature until firm, or pop it in the refrigerator to speed things up.
Once set, lift the fudge from the pan using the parchment paper or foil, cut it into squares, and enjoy!