Combine in a heavy saucepan:
1 pound butter (not margarine)
2 cups light corn syrup
2 cans sweetened condensed milk
Heat over medium-low. This burns easily, so stir constantly.Stir until all is melted and blended.
Then stir in:
2 pounds brown sugar
2 teaspoons vanilla
Continue stirring until mixture starts to boil. Let boil about 1 minute. Remove from heat. Can store in refrigerator for 2 - 3 weeks.
OH YEAH--this makes a LARGE amount as you can see in the photos--you can easily cut it down.