Heat oil and a few popcorn kernels in a large, heavy-bottomed pot over high. When kernels pop, add 1/2 cup kernels, cover, and remove from heat; let stand 30 seconds. Return to high heat; cook until popping stops, about 2 minutes. Transfer to a bowl. Toss with butter, cheese, Tajín, and salt. Add more Tajín to taste, and cayenne for extra heat, if desired.