Line an 8x8 inch baking pan with parchment paper or foil, leaving overhang on sides. Lightly butter the lining.
In a heavy-bottomed saucepan over medium heat, combine butter, granulated sugar, brown sugar, heavy cream, and salt. Stir constantly until mixture comes to a full boil.
Once boiling, stop stirring and insert candy thermometer. Cook without stirring until temperature reaches 234°F to 236°F (soft-ball stage), about 4-5 minutes.
Remove from heat and let cool without stirring for 20-30 minutes until mixture reaches 110°F.
Add vanilla extract. Beat with wooden spoon until mixture thickens and loses glossy sheen.
Gradually stir in sifted powdered sugar until smooth. Fold in toasted pecans.
Pour into prepared pan and spread evenly. Cool at room temperature for at least 2 hours or refrigerate until firm.
Lift fudge from pan using overhang. Cut into 1-inch squares with sharp knife, wiping blade between cuts.