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 25-30 minutes or until a 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 with frosting; sprinkle with peanuts. Cool completely.Yield: 15 servings.
Originally published as Chocolate-Peanut Butter Sheet Cake in Taste of Home February/March 2015, p64