In a large mixing bowl, combine butter and brown sugar; beat in eggs, chocolate and vanilla. Combine the 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 degrees for 35-40 minutes or until a toothpick inserted near the center comes out clean. Cool for 10 minutes before removing from pans to wire racks to cool completely.
For frosting, cream the butter, peanut butter, confectioners' sugar, milk and vanilla in a mixing bowl until smooth; set aside. Split each cake into two layers. Place a bottom layer on a serving plate; spread with about 1/2 cup frosting.
Repeat layers twice. Top with the remaining cake. Frost top and sides of cake. Gently press peanuts into sides of cake.
*Reduced-fat or generic brands of peanut butter are not recommended for this recipe.