Hmmm, Jeruba that shouldn’t have made a difference. Although, logically, yes, the cookies are not actually in contact with the full heat of the metal pan and the white parchment may be reflecting heat away from the whole business. You probably do need to increase the baking time, just a wee bit. I don’t think I would increase the temperature however. I use parchment a lot (but, with recipes and techniques adapted to food service) and have never really thought about this before. I don’t know if this was helpful or not!