Start to melt butter over low to medium heat. Just before butter is melted, add all dark brown sugar at once and stir with wooden spoon until sugar is uniformly wet.

After it has cooked 10 minutes let it rest and cool till it reaches room temperature

When butterscotch liquid is room temperature, take a small taste. Whisk in half the salt and vanilla extract Add more salt and vanilla extract until it taste like butterscotch.