In a heavy saucepan, melt butter over medium heat.
Add sugar and salt. Stir constantly.
The mixture will separate at first—keep stirring, it will come together.
Cook until it reaches 300°F (hard crack stage) and is a deep amber color (about 10–15 minutes).
Remove from heat.
Quickly stir in vanilla (it will bubble—normal).
Immediately pour onto prepared pan and spread evenly.