In a large bowl, cream butter and sugar until fluffy, beat in eggs, chocolate and vanilla. Combine flour, baking soda and salt; add to creamed mixture alternately with sour cream, beating well after each addition. Gradually beat in water.
Pour into 2 greased and floured 9 inch baking pans. Bake at 350 degrees for 35-40 min 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: In a large bowl cream the butter, peanut butter and powdered sugar. Beat in milk and vanilla until smooth. Set aside.
Cut each cake horizontally into 2 layers. Place bottom layer on a serving plate, top with 1/2 cup frosting. Repeat layers twice. Top with remaining cake layer. Frost top and sides of cake with remaining frosting. Gently press peanuts into sides of the cake.