In saucepan over medium – low heat, add 6 tablespoons of butter and allow to melt. Add 2/3 cup brown sugar and mix well. Pour in 14 oz can sweetened condensed milk and bring to a boil stiring constantly.
Boil for 5 to 10 minutes, or until it becomes thick.
Once thick, mix 1 teaspoon of vanilla. Remove from heat and pour into prepared crust.
Allow the toffee in the crust to cool for 1 hour in the fridge.