Just the way Mom made it…
1 cup light brown sugar, firmly packed
2/3 cup light corn syrup
1/4 cup butter
2/3 cup evaporated milk
1/8 teaspoon baking soda
1 teaspoon vanilla
Combine the sugar, syrup and butter and cook, stirring constantly over a medium heat, until the sugar has dissolved and the mixture has come to a full boil. Stop stirring and boil for exactly 1 minute more.
Remove from the heat, without stirring!!! and let stand for 5 minutes.
In a second bowl, combine the milk, baking soda and vanilla. Stir into the the slightly cooled sauce. Serve warm. Allow to cool before storing in a jar. To reheat, place the jar in a bowl of hot water until the butterscotch has warmed through. You can also use a microwave, use a low setting and heat slowly.