One of New York City’s most impressive turn-of-the-century Gilded Age mansions—located on 5th Avenue at 79th Street—houses the Ukrainian Institute. The mansion was built in 1899 for Isaac D. Fletcher—businessman, art collector and museum benefactor. It was designed in the elaborate Châteauesque style by C.P.H. Gilbert, who was known for many notable palatial residences for the wealthy. Châteauesque, inspired by…