For Cake-In a mixing bowl, cream brown sugar and butter.Add eggs, one at a time, beating well after each addition. Beat on high speed until light and fluffy. Blend in vanilla.
Combine flour, cocoa, baking soda and salt; add alternately with sour cream to creamed mixture. Mix on low just until combined. Stir in water until blended. Pour into three greased and floured 9-in. round baking pans.
Bake at 350 degrees F for 35 minutes. Cool in pans 10 minutes; remove to wire racks to cool completely.
For Frosting- In a medium saucepan, melt butter and chocolate over low heat. Cool several minutes. In a mixing bowl, combine sugar, sour cream and vanilla. Add chocolate mixture and beat until smooth.
Frost cooled cake.
