Preheat air fryer to 400F, line basket with aluminum foil.

In a small bowl, stir together the popcorn kernels and oil, then spread out into a single layer on the tinfoil. Cover the kernels with a mesh or wire rack -- this part is important to make sure they don't fly up into the heating coil.

Air fry for 8 minutes, or until the popping sounds slow down, then remove popcorn and return unpopped kernels to the air fryer and cook an additional 3 minutes.

Discard any unpopped kernels.

Drizzle melted butter and sprinkle salt over popped popcorn.