Peanut Butter Sheet Cake

This is a fun and tasty twist on the usual chocolate sheet cake, and since you make it with a boxed cake mix, it's even easier to throw together and enjoy. Everyone loved it, and it will for sure be joining my regular rotation of easy desserts. Yum!

Peanut Butter Sheet Cake
1/2-3/4 cup creamy peanut butter
1/2 cup butter, softened
4 eggs
1 (18 1/2 oz.) yellow cake mix
2/3 cup water
frosting:
1/2 cup butter
1/2 cup peanut butter
6 TBSP. milk
1 tsp. vanilla
3 3/4 cup powdered sugar

Preheat oven 325* F. Grease large cookie sheet and set aside.
In a large mixing bowl, cream the butter and peanut butter together. Add the eggs, one at a time, beating until well mixed. Add the cake mix and water alternately, beating each addition until well mixed and creamy. Pour the batter into the greased cookie sheet, spreading it evenly. Bake @ 325* F for 25-35 minutes, until cake tests done. Place on wire rack to cool.
For the frosting, in a large saucepan over medium high heat, melt the butter, peanut butter and milk, stirring to mix. Bring to a boil and let cook for 1 minute (it will look like it has separated a bit, but this will correct itself once you beat it with the powdered sugar). Remove from heat and add the vanilla and powdered sugar. Mix with an electric mixer for the smoothest and best result. When the frosting is well mixed and creamy, frost the warm cake with the warm frosting. Let the frosted cake cool completely, cut, then serve.