Prepare the pan:
Line an 8x8-inch square baking pan with parchment paper or aluminum foil, leaving an overhang for easy removal.
Lightly butter the lining.
Melt the chocolate mixture:
In a medium saucepan over low heat, combine the chocolate chips, sweetened condensed milk, and butter.
Stir constantly until the mixture is completely melted and smooth (about 5–7 minutes).
Add flavoring:
Remove from heat and stir in vanilla extract and salt.
Pour and set:
Pour the fudge mixture evenly into the prepared pan.
Smooth the top with a spatula and add optional toppings if desired.
Chill:
Refrigerate for at least 2 hours, or until firm.
Cut and serve:
Lift the fudge out using the parchment overhang.
Cut into small squares with a sharp knife.