Is it my oven?
Every type of cookie I've ever made, with or without parchment paper, different temperatures, etc. Not specific to top rack or bottom rack. Always a brown bottom. They taste great, and I barely cook them so the bottom doesn't BURN so there's no burnt situation, usually. Just browner than the rest. This displeases me aesthetically.
But in the event I leave them in 1/2 a minute too long, instead of just getting a little crunchy the bottoms just get SCORCHED.
I just made some awesome banana chocolate chip cookies (just a little darker on the bottom).