Preheat oven to 350 degrees. Grease a 13-x9-inch cookie sheet. 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 until just moistened.
In a small bowl, whisk eggs, sour cream, and vanilla until blended; add to flour mixture, whisking constantly. Pour onto cookie sheet.
Bake 25-30 minutes or until toothpick inserted in center comes out clean. Prepare frosting while cake is baking.
In a large bowl, beat confectioners’ sugar, peanut butter, milk, and vanilla until smooth. Remove cake from oven; place on a wire rack. Immediately spread frosting onto cake; sprinkle with peanuts. Cool completely.