Place the Cheddar powder, buttermilk powder, salt and cornstarch in a spice grinder or mini food processor and whir for about 10-15 seconds to blend evenly. Set aside.
Pour the vegetable oil into a 3 or 4 quart heavy-bottomed stockpot and add 2 or 3 of the popcorn kernels. Cover the pot and heat over medium heat until you hear 1 or 2 of the kernels pop. Add the remaining kernels in an even layer and cover again. Cook, shaking gently and frequently to evenly distribute the hot oil among the kernels as they pop. Remove from heat once the popping sounds slow to a crawl. Transfer the cooked popcorn to a large bowl.
Melt the butter in a small saucepan over low heat; don't let it bubble or spurt. Pour the butter over the popcorn and toss to evenly combine. Add the cheese topping to the buttered popcorn and toss or shake to combine.
Store the popcorn at room temperature in an airtight container for up to a week.
If you don't want to make your own popcorn on the stove, substitute 10 cups of air-popped popcorn for the kernels and oil!