In large mixing bowl, combine butter and brown sugar; beat in eggs, chocolate and vanilla. Combine flour, baking soda and salt; add to creamed mixture alternately with the sour cream.
Gradually beat in water. Pour into two greased and floured 9-inch round baking pans. Bake at 350° 35 to 40 minutes or until a toothpick inserted near center comes out clean. Cool 10 minutes before removing from pans to wire racks to cool completely.
Frosting: Cream butter, peanut butter, confectioners? sugar, milk and vanilla in mixing bowl until smooth; set aside. Split each cake into two layers. Place bottom layer on serving plate; spread with about 1/2 cup frosting.
Repeat layers twice. Top with remaining cake. Frost top and sides of cake. Gently press peanuts into sides of cake.