Combine cake mix, pudding, eggs and buttermilk. Blend at low speed until moistened (30 seconds) then on medium speed for 2 minutes. Pour into cake pans and bake for 25-30 minutes.
Grate about 2/3 of Hershey bar, curl remaining 1/3 and set aside.
Combine cream cheese, powdered sugar and granulated sugar. Mix well. Add shredded chocolate and fold in Cool Whip.
Frost cooled cake, garnish with chocolate curls.