Preheat oven to 350. Grease and flour a 9x13 pan. 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.