Combine chili powder, paprika and salt in a small bowl. In a large covered pot over medium-high heat, warm oil. Add a few kernels of corn and shake until they pop. Add remaining kernels, cover and cook, shaking pot often, until 4 to 5 seconds lapse between pops. Remove from heat and shake until popping stops. (Time varies depending on how fresh the corn is, what size pot you use and amount of heat. Watch carefully to prevent scorching.)
Immediately pour popped corn into a large bowl and toss with melted butter. Place spice mixture in a small sieve and sift over buttered popcorn. Toss and serve.