Melt the butter in a heavy gauge small saucepan over medium heat.
Stir in the brown sugar, evaporated milk and salt, whisking well.
Bring the sauce to a boil, and let it bubble for about 5 minutes whisking periodically. Remove from heat and whisk in the vanilla extract.
Taking care not to burn yourself, taste and adjust seasoning with salt and vanilla to your preference.
This sauce thickens as it cools and can be stored in an airtight container in the refrigerator. I like to reheat it in the microwave before use.
Butterbeer:
Combine cream soda with warm butterscotch sauce. Return to bottle if desired.
I prefer the beverage to be warm, because the butter content tended to solidify when cooled. While this is already a very rich drink, you may wish to garnish with whipped cream.
Notes
Butterscotch Sauce adapted ever so slightly from Smitten Kitchen. The original recipe calls for heavy cream, and I was pleased to find that evaporated milk substitutes quite nicely.