Preheat the oven to 350 degrees F. Grease and lightly flour two 9-inch cake pans

For the cake: Sift together the flour, cocoa, baking soda, baking powder and salt and set aside. With an electric mixer, cream the butter and sugar until fluffy, about 5 minutes. Add the eggs and vanilla and beat on high speed for 3 minutes. Add the flour mixture in stages with 1 1/3 cups water, beginning and ending with the flour.

Divide the batter evenly between the pans and bake for 30 to 35 minutes. Turn the layers out onto racks that have been sprayed with cooking spray.

For the caramel icing: Mix 3 cups of the sugar and the milk in a heavy 3-quart saucepan. Bring slowly to a boil and keep it hot. Caramelize the remaining cup of sugar in an iron skillet. Do this by cooking over medium-high heat and stirring and scraping the pan with a flat-edged spatula as the sugar melts. Continue to cook until the syrup turns to medium or dark brown in color. This occurs at about 320 degrees F to 350 degrees F on a candy thermometer. Do not scorch the syrup. Stream the syrup into the boiling sugar and milk mixture and cook to the soft ball stage, about 238 degrees F. Add the butter, vanilla extract and baking soda.

Pour the hot mixture into the bowl of an electric mixer and beat as it cools until the icing is creamy, 15 to 20 minutes. Watch the icing because it will go from runny to too thick very quickly. Spread on the cake layers while the icing is still warm. If it becomes too stiff, add a few drops of hot water.