Cream brown sugar and butter in a large mixing bowl.
Add eggs, one at a time and beat well after each addition.
Turn mixer on high and beat until mixture is light and fluffy. Blend in vanilla.
In a separate bowl, combine flour, cocoa, baking soda, and salt.
With mixer on low speed, add dry ingredients alternately with sour cream to butter mixture.
Mix until just combined.
Blend in water.
Preheat oven to 350° F.
Grease and flour three 9 inch round baking pans.
Bake the cake for about 35 minutes.