Add 1/2 cup unpopped popcorn kernels into the air popper. (1/2 cup is the standard max amount of kernels for most air poppers. If yours varies, you may have to pop the popcorn in two batches.)
While the popcorn is popping, heat the butter and maple syrup in a small pot over medium low heat. Once it comes to a boil, reduce it to a simmer and simmer for 2 minutes.Watch it carefully and turn down the heat if needed so it doesn't burn. It will begin to foam as it heats.
Remove the maple syrup mixture from the heat, and stir in the salt.
Dump half of the maple syrup mixture over the popped popcorn.
Gently stir until well coated.
Add in the remaining maple syrup mixture and stir.