In a large mixing bowl, cream butter and brown sugar.
Add eggs, one at a time, beating well after each addition.
Beat on high speed until light and fluffy.
Blend in vanilla. In a separate bowl combine flour, Saco Premium Baking Cocoa, baking soda and salt; add alternately with sour cream to creamed mixture.
Mix on low speed just until combined.
Stir in water until blended.
Pour into prepared cake pans.
Bake for 30-35 minutes or until inserted toothpick comes out clean.
Frosting
In a medium saucepan, melt butter and chocolate over low heat, stirring often.
Cool slightly. In a mixing bowl, combine powdered sugar, sour cream and vanilla.