Line 2 9 in. cake pans with waxed or parchment paper, and lightly grease and flour them for easy cake removal.
Melt 3 squares unsweetened chocolate in microwave or double boiler and set aside.
Sift together and set aside:
2 1/4 cups sifted flour
2 tsp. baking soda
1/2 tsp. salt
In mixer, beat 4 oz. sugar with 2 1/4 cups packed brown sugar.
Add, and beat in 3 large eggs.
Add melted chocolate and 1 1/2 tsp. vanilla.
Stir in dry ingredients alternately with 8 oz. sour cream, beginning and ending with flour to incorporate fat.
Stir in 1 cup boiling water.
Pour into prepared cake pans and bake at 350 for 25 minutes or until cake pulls away from sides of pan and tests clean in center.