1. Line 8-inch square baking pan with plastic wrap; smooth out wrinkles. In heavy 2-quart saucepan, melt chocolate with condensed milk over medium-low heat, stirring constantly, until smooth. Remove from heat.
2. Stir in walnuts, vanilla, and salt. Scrape chocolate mixture into prepared pan; spread evenly. Refrigerate until firm, about 3 hours.
3. Remove fudge from pan by lifting edges of plastic wrap. Invert fudge onto cutting board; discard plastic wrap. Cut fudge into 8 strips, then cut each strip crosswise into 8 pieces. Layer between waxed paper in airtight container. Store at room temperature up to 1 week, or refrigerate up to 1 month.