The other, even better answer to the problem is to fix the heat leak, assuming you can access that point in the attic.
A common thin-spot in insulation is at the top plate of the studwall, where they don't blow in fiber or stuff batts that might obstruct soffit ventilation. Sometimes wind-washing will move blown insulation out of those areas creating thin spots too. If you loosely cut'n'cobble sections of rigid XPS foam (pink, blue green, whatever) to at least 4" depth (R20) beween the joists directly on the top plate of the studwall,leaving a minimal 1" of vent space between the XPS and the roof deck, and use 1-part expanding foam (eg Dow Great Stuff) between the sides of the XPS and joists, then re-install the attic insulation right up against your stack, that section of ceiling will run much warmer, spending more time above the dew point of the bathroom air, with many fewer condensing hours.
While your up there, assess the depth and condition of the attic insulation. If its less than 10" from the top of the insulation and the ceilng, an overblow of 6" of cellulose to even it up and fill in all gaps would be cost effective. (Even more is warranted if you heat with oil, propane, or electricity.)