DoubleTree Magnificent Mile: January 2016 report by Diamond states voucher for continental buffet only with option to upgrade to full for $7.
Confirmed. The continental buffet is mediocre. Upgrade is $7+tax. Coupon said "Gold and Diamond" so I'm presuming Gold gets the same, but I can't be sure.
Also the full name for the hotel is "DoubleTree by Hilton Chicago Magnificent Mile" (at a minimum needs Chicago added)