– Preheat oven to 375°. Grease and flour rectangle baking sheet.
– 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.