Ken Moelis’ two-year-old investment bank, Moelis & Co., has signed a lease for 90,000 square feet at 399 Park Avenue between 53rd and 54th streets. This announcement comes during a challenging Manhattan office leasing environment, which has been crippled by tumbling asking prices and slow leasing velocity. In the last year, 399 Park Avenue saw Citigroup and Legg Mason return a combined 194,000 square feet to the market. But, as the New York Observer points out, Moelis is in a unique position to make this deal — it is one of the few investment banks that has grown during the recession. Since its founding in July 2007, Moelis’ employee roster has grown from just 10 to 230. In its last home, 245 Park Avenue, Moelis had a sublease on just 30,000 square feet, although Rohini Pragasam, a spokesperson for Moelis, would not disclose whether that was the total amount of space the bank was leasing.