The Moyhu NCEP/NCAR index for December was 0.621°C, up from November's 0.513°C, and ahead of the previous record - October 2015 at 0.567°C, which itself was a big jump on the previous record. It was an eventful month, with another extraordinary peak early, then mid-month the steepest plunge in recent years (but only to values which would have been high a few months ago) and then back to hot at the end.
There were some late data troubles - a data pause from NCEP for three days, and at my end, I have some end of year issues to fix. But they don't affect the results posted.
Adjusted to the 1951-1980 baseline of GISS, that would give a month anomaly of 1.18°C. The current record there is October at 1.06°C. I would expect that GISS might be a bit lower than 1.18, but still the hottest month in the record. Which makes 2015 even more securely the hottest year.