In a small bowl, stir together the sesame seeds, mustard, and salt. Set aside.
Using an air popper, pop the corn kernels into a large bowl. Meanwhile, combine the butter and sesame oil in a small bowl. As the large bowl begins to fill with popcorn, alternately drizzle the butter–sesame oil mixture over the popcorn and sprinkle it with the sesame seed mixture, occasionally tossing to coat.
When the popcorn stops popping, use your hands to toss everything together and ensure that the ingredients are evenly distributed. Serve immediately.