Line an 8x8-inch square pan with parchment, leaving overhang for removal.

Combine chocolate, condensed milk, and butter in a medium saucepan over low.

Stir constantly 5-7 minutes until chocolate fully melts and mixture is smooth.

Off heat, stir in vanilla and salt; fold in optional nuts.

Spread fudge evenly in pan with a spatula.

Refrigerate at least 2 hours or until completely firm.

Lift out by parchment; cut into 1-inch squares and store airtight in refrigerator up to 2 weeks.