Add white and brown sugar to a medium saucepan with milk and salt.
Stir together with a wooden spoon or silicon spatula and heat over medium heat until boiling, then stop stirring and continue heating until the temperature reads 234 degrees when checked with a candy thermometer.
Remove sugar mixture from heat and pour the hot mixture into the bowl of a stand mixer along with peanut butter, vanilla extract, and butter.
Set the mixer on medium-low and let mix for approximately seven minutes. The fudge is done mixing when it has thickened, but still remains pourable.
Pour the fudge into a medium pan lined with wax paper to cool. Let cool at room temperature for 2-3 hours, then slice and enjoy.
Notes
Optional: While cooling you can place chocolate, nuts, or crushed candy bars on top of the fudge for additional flavor and texture.