Preheat oven to 350 degrees. Grease and flour a 9 by 13 cake pan. Set aside. Sift together the cake flour, baking soda and salt and set aside. Beat the butter and sugar together in a large mixing bowl. Blend in the eggs and beat until light and fluffy. Stir in the vanilla and melted chocolate. Alternately add the dry ingredients and the sour cream to the butter mixture. Beat well after each addition. Stir in boiling water. (Batter will be thin) Divide cake batter in half. Pour one half into the prepared cake pan. Bake 10 - 12 minutes, or until firm to the touch, and remove from the oven. Meanwhile, prepare the filling. Mix butter, caramels, and condensed milk in a sauace pan and melt over low heat, stirring often. Remove from the stove and pour the mixture over the baked half of the cake. Pour the remaining cake batter over the caramel mixture and return to the oven. Bake for 15 - 20 minutes or until firm to the touch. When cool, serve with ice cream, whipped cream or frost with your favorite chocolate frosing. NOTE: melt chocolate in double boiler over simmer (not boiling water)