Line the bottom of an 8 inch square baking pan with waxed or parchment paper. In a medium-size metal bowl set over a pot of simmering water, combine the chocolate, butter, milk and salt, stirring frequently until melted and combined. Fold in the mixed nuts. Pour the chocolate mixture into the lined baking pan and chill, uncovered, for at least 4 hours. Run a knife around the edges of the pan and invert Fudge onto a cutting board. Remove paper and cut fudge into 1 inch squares.