Line an 8-inch square baking pan with a strip of wax or parchment paper, leaving an overhang on two sides.
In a double boiler or a medium heatproof bowl set over (not in) a saucepan of barely simmering water, heat the chocolate, condensed milk, butter, and salt, stirring occasionally, until smooth. Stir in the walnuts.
Pour the chocolate mixture into the prepared pan and chill, uncovered, until firm, at least 4 hours and up to 3 days.
Run a knife around the edges of the pan to loosen the fudge. Using the parchment overhang, lift the fudge out of the pan. Cut into 64 1-inch squares. Serve chilled.
Storage suggestion: Keep the fudge refrigerated, in an airtight container, for 2 weeks.
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