Preheat oven to 350°. Grease and flour three 9-in. round baking pans.

In a large bowl, cream butter and brown sugar until light and fluffy. Add eggs, one at a time, beating well after each addition. Beat in vanilla. In another bowl, whisk flour, cocoa, baking soda and salt; add to creamed mixture alternately with sour cream, beating well after each addition.

Stir in water until blended.

Transfer to prepared pans. Bake until a toothpick comes out clean, 30-35 minutes. Cool in pans 10 minutes; remove to wire racks to cool completely.

For frosting, in a metal bowl over simmering water, melt butter and chocolates; stir until smooth. Cool slightly.

In a large bowl, combine confectioners' sugar, sour cream and vanilla. Add chocolate mixture; beat until smooth. Spread frosting between layers and over top and sides of cake. Refrigerate leftovers.