1Heat oven to 350°F. Spray bottom only of 15x10x1-inch pan with cooking spray.
2In large bowl, beat cake ingredients with electric mixer on low speed 30 seconds, then on medium speed 2 minutes, scraping bowl occasionally. Pour batter into pan.
3Bake 18 to 22 minutes or until toothpick inserted in center comes out clean. Cool completely on cooling rack, at least 1 hour.
4In 2-quart saucepan, heat butter, peanut butter and milk over medium-low heat, stirring constantly, until butter and peanut butter are melted. Heat to boiling; remove from heat. Stir in vanilla. Working quickly, add powdered sugar, 1 cup at a time, beating with whisk until incorporated and icing is smooth. Immediately spread icing over cake. Sprinkle with peanuts. Store loosely covered.