Heat oven to 300. Spray a 9" x 13" baking pan w/ cooking spray. In bowl, combine flours, sugar, cocoa powder, salt, baking soda and baking powder.
In another bowl, beat eggs. Beat in applesauce or oil, coffee and vanilla. Beat in flour mix, alternately w/ buttermilk. Pour into pan, bake 1 hr, or until a toothpick inserted in center comes out clean. Let cool.
In bowl, beat butter and peanut butter until smooth. Beat in powdered sugar. Slowly beat in milk until spreadable. Beat 3 mins. Frost cooled cake.