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.
