In heavy saucepan, over medium heat, melt chocolate chips and butter. Add sweetened condensed milk and salt. Stir until smooth. Do not overheat.
Remove from heat as soon as it's melted. Stir in vanilla (and nuts, if using). Spread evenly into parchment paper lined 9-inch square pan or loaf pan. Chill 3 hours or until firm.
Turn fudge onto cutting board; peel off paper and cut into squares. Store tightly covered at room temperature or in refrigerator.
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Notes
If you like thicker cubes of fudge, use the loaf pan for this fudge recipe. Or if you prefer a thinner square, use the 9-inch square pan.