Line an 8-inch square pan with foil, wax, or parchment paper and grease the inside with softened butter.
Combine the chocolate chips, condensed milk, and salt in a medium saucepan. Stir over medium heat until the chocolate has melted and the mixture is smooth. Remove from the heat and stir the vanilla and the nuts, if using, into the chocolate mixture. Pour it into the prepared pan and smooth the top.
Let the fudge sit in a very cool dry place until the fudge has set, or if your room is warm, chill, but only after it comes to room temperature, about 1 hour more.
Cut into 1-inch squares and serve. Store in an airtight container for up to a week or in the fridge if it’s not all that cool in your kitchen. Let come to room temp just before serving. Makes 64 pieces.
Notes
Small squares are the way to go, perfect for nibbling and getting your chocolate fix. I’m not saying you can’t have more than 1 piece, but I am saying that this fudge is rich.
If your room is warm, you can chill the fudge, but only after it comes to room temperature, about 1 hour in the pan.
Store in an airtight container for up to a week or in the fridge if it’s not all that cool in your kitchen. Let come to room temp just before serving.
Putting the fudge in the freezer could cause it to become grainy. (But if you have extra, you might as well give it a go — it's also kind of fun to nibble when still frozen.)