Line an 8x8-inch pan with aluminum foil. Grease foil with butter.
In a medium, heavy-bottomed saucepan, combine butter, both sugars, cream, and salt.
Bring to a rapid boil over medium heat, stirring constantly. Cook, without stirring, until a candy thermometerreads 234° (soft-ball stage). Remove from heat. Add vanilla to pan (do not stir).
Cool, without stirring, to 110°, about 30 minutes.
Beat with a spoon until fudge just begins to thicken.
Gradually stir in confectioners' sugar until smooth; add nuts and continue stirring until fudge becomes very thick and just begins to lose its sheen. Immediately spread into prepared pan. Cool.
Notes
Note: Sift confectioners' sugar and then measure it to make sure that there are no lumps and you get a correct measurement.