Combine first 4 ingredients in a large bowl; stir in creamy peanut butter, buttermilk and beaten eggs.
Melt butter in a heavy saucepan over medium heat. Whisk in 1 cup water and cocoa. Bring to a boil, whisking constantly. Remove from heat.
Stir cocoa mixture into flour mixture. Pour batter into a lightly greased 15- x 10-inch jellyroll pan.
Bake at 325° for 20 to 25 minutes or until a wooden pick inserted in center of cake comes out clean. Spread Fudge Frosting evenly over warm cake.
Garnish with miniature peanut butter cup candies, halved, if desired.
Find recipes with ingredients that you have on hand.