• 4. To assemble whoopie pies, put filling into a zip-top bag and snip off one corner. Place chopped walnuts into a shallow dish or pie plate. Flip over one carrot cake cookie and pipe about ¼ cup of frosting onto center of flat side. Top with another cookie, flat-side down, and press gently to distribute frosting to edges. Roll edges of pie in the chopped walnuts and set aside. Repeat with remaining cookies and frosting.