The nation's longest street is becoming a regular cake walk — at least along Cupcake Row, where Mulberries, an emigré from Platt Park, has joined the Shoppe and Lovely Cupcakes. Ah, such sweet suffering. Mulberries' claim to fame is pure whimsy: topsy-turvy cakes of all sizes, decorated with colorful buttercream bows, blossoms, dots, ladybugs and blobs, or cupcake towers like bouquets of stunning rosettes. We say welcome to the 'Fax: Old South Pearl Street's loss is East Colfax's gain.