Make the mini meringues: Arrange two racks in the center of the oven and preheat the oven to 200°F. Line two baking sheets with parchment paper.
In a medium bowl, using a whisk or handheld mixer, beat the egg whites and a pinch each of cream of tartar and salt until the egg whites are foamy, about 1 minute. Increase the speed to high (or really get your arm going) and gradually add the sugar while beating until the egg whites form stiff, glossy peaks, about 5 minutes more.
Transfer the meringue to a small piping bag fitted with a 1⁄4-inch star tip. Pipe 3⁄4-inch mounds of meringue (about the size of a popped piece of popcorn) onto the prepared baking sheets (you should get 125 to 150 meringues total). Bake until they are completely dry and crisp, 35 to 40 minutes, rotating the baking sheets front to back and top to bottom halfway through the cooking time. Remove from the oven and set the baking sheets on a wire rack. Let the meringues cool completely. (The meringues can be cooled and stored in an airtight container at room temperature for 2 to 3 days.)
Make the popcorn: In a large heavy-bottomed pot or Dutch oven, heat the canola oil over medium-high heat. When the oil shimmers, add the popcorn kernels and give them a good shake. Cook, stirring continuously, for 1 minute, then cover the pot with a lid; you should hear the kernels start popping after 30 seconds to 1 minute. Cook, shaking the pot often to prevent the kernels from burning, until the sound of the popping kernels slows down significantly, 3 to 4 minutes. Turn off the heat but keep the lid on until the popping stops completely.
Remove the lid, drizzle in the butter, and season liberally with salt. Put the lid back on and give the pot a few good shakes so that the popcorn gets evenly coated with the butter and salt.
Transfer the warm popcorn to a large bowl and gently toss with the cooled mini meringues. Serve within a few hours so the popcorn doesn’t get stale.