Preheat oven to 350 degrees.
Grease and flour a 9 x 13 inch pan.
Set aside.
Sift together the cake flour, baking soda, and salt; set aside.
Beat butter and sugar together in a large mixing bowl.
Blend in eggs and beat until light and fluffy.
Stir in vanilla and melted chocolate.
Alternately add dry ingredients and 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.
Remove from oven.
While cake is baking, prepare filling.
Over low heat melt butter, caramels, and condensed milk in a sauce pan, 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 oven.
Bake 15 - 20 minutes or until firm to the touch.
When cool, serve with ice cream, whipped cream or frost.