THERE MAY BE no perch in New York City better for an ice cream sundae than one of the stools at the marble counter of Brooklyn Farmacy & Soda Fountain. The 1920s apothecary-turned-21st-century-soda-fountain triggers powerful nostalgia for the prohibition-era "golden" age of the sundae, when countless bars became soda fountains, and many bartenders soda jerks.