Cake:
Heat oven to 350-degrees. Spray bottom of 9 x 13 pan. In large bowl, beat cake mix, water, peanut butter, butter, and the 3 eggs on low speed for 30 seconds.

Beat on medium speed 2 minutes. Scrape bowl. Bake at 35 – 40 minutes. Cool 10 minutes. Run knife around cake. Cool an additional 1 hour before frosting.

Frosting:
Blend on low speed the peanut butter and butter. Add sugar, milk and vanilla. Blend on low until sugar is combined, about 1 minute. Increase speed to medium until frosting lightens and is fluffy. If too thick beat in up to 1 Tbsp more milk.