Sift together the flour, baking soda and salt. Set aside.
In a large bowl, beat 1/2 cup butter or margarine and 2 1/2 cups brown sugar until well mixed. Add eggs one at a time. Beat in the vanilla and melted chocolate squares. Add 1/2 the sour cream and then 1/2 the dry ingredients to the butter mixture until well blended.
Add the remaining sour cream and dry ingredients to the batter. Stir in boiling water.
Bake in a greased 9 X 13 inch pan for 35 minutes.
Let cool 10 minutes before icing.
To Make Caramel Icing: First melt 1/2 cup butter in a saucepan. Stir in 1 cup brown sugar and boil about 2 minutes. Stir in 1/4 cup milk and bring to a boil. Place pan in cold water and stir in the confectioners' sugar. Continue stirring until smooth. Spread over still warm cake.