sweetened condensed milk
Preheat oven to 350; grease and flour a 9x13 pan and set aside.
Sift together cake flour, baking soda and salt; set aside.
Beat butter and sugar together in a large mixing bowl; blend in the eggs and beat until light and fluffy.
Stir in vanilla and melted chocolate; alternately, add the dry ingredients and the sour cream to the butter mixture.
Beat well after each addition; stir in the boiling water. (Batter will be thin.)
Divide cake batter in half; pour one half into prepared cake pan.
Bake 10-12 minutes or until firm to touch; remove from oven.
Meanwhile, prepare filling; mix the butter, caramels and sweetened condensed milk in a sauce pan and melt over low heat, stirring often - Can be melted in the microwave.
When melted, pour the mixture over the half baked cake; pour the remaining cake batter over the caramel mixture and return to the oven.
Bake for 20-25 minutes or until firm to the touch.
Cool and frost or serve with whipped cream or a la mode.