Preheat oven to 350°. Grease a 13x9-in. baking pan., In a large bowl, whisk flour, sugar, baking soda and salt. In a small saucepan, combine water, butter, peanut butter and cocoa; bring just to a boil, stirring occasionally. Add to flour mixture, stirring just until moistened., In a small bowl, whisk eggs, sour cream and vanilla until blended; add to flour mixture, whisking constantly. Transfer to prepared pan. Bake until a toothpick inserted in center comes out clean, 25-30 minutes., Meanwhile, prepare frosting. In a large bowl, beat confectioners' sugar, peanut butter, milk and vanilla until smooth., Remove cake from oven; place pan on a wire rack. Immediately spread cake with frosting; sprinkle with peanuts. Cool completely.