Preheat oven to 350°. Lightly spray a 13x9-inch baking pan with baking spray with flour.
In a large bowl, whisk together flour, sugars, baking powder, baking soda, and salt.
In a medium bowl, whisk together milk, peanut butter, sour cream, oil, eggs, and vanilla until almost combined. Add milk mixture to flour mixture; beat with a mixer at medium speed until smooth, stopping to scrape sides of bowl. Reserve two cups of batter.
In a small bowl, stir together cocoa and ⅓ cup boiling water until smooth. Mix into reserved batter until combined.
Spread 2 cups peanut butter batter into bottom of prepared pan; drizzle with chocolate batter. Drizzle remaining peanut butter batter over chocolate batter. Using the tip of a knife, swirl batters together. Gently tap pan on counter twice to release air bubbles.
Bake until a wooden pick inserted in center comes out clean, about 25 minutes. Let cool completely on a wire rack. Spread Peanut Butter Frosting onto cake, and sprinkle with peanuts.
Recipe by Taste of the South at https://tasteofthesouthmagazine.com/peanut-chocolate-marble-cake/