Air poppers are a great tool for popping corn. They're cheap and quick, but they won't apply the butter for you. I used to melt a little butter in a bowl and then pour it over the popped corn in as thin a stream as I could. I would then grab a spoon as quickly as possible to toss things around. It never worked. Invariably, I always ended up with kernels that were so coated in butter they were soggy and some that were naked as the day they were born. I knew there had to be a better way.
And there is. All you have to do is brush the stuff on. I use a pastry brush to apply it. For a half cup of unpopped kernels, I only need to use about 1 tablespoons of butter and I am completely satisfied with the flavor.
Just blot the popped kernels with the brush, tossing them in the bowl as you go to get all sides covered. You'll end up with a perfectly even coat every time. At this point, you can add a sprinkling of popcorn salt and curl up on the couch with your favorite movie while happily crunching away. Yum!