If the bottoms of your cookies are burning, it's probably because the cookie sheet is getting too hot. There are a couple of things you can do to keep this from happening.

First, you should move an over rack directly under the rack you are baking on and put aluminum foil on that rack. This will keep the cookie sheet from getting the direct heat from the burner.

Second, give your cookie sheet time to cool off between batches of cookies. The easiest way to do this is to get several cookie sheets so that you can rotate through them as you bake.