Heat broiler, with rack set 4 inches from heat. Line a rimmed baking sheet with aluminum foil. In a small bowl, combine chile powder, 1 teaspoon salt, and 1/4 teaspoon pepper. Place pork on prepared baking sheet; rub with spice mixture.
Broil pork until an instant-read thermometer inserted in thickest part registers 145 degrees, 14 to 16 minutes, turning twice during cooking. Let pork rest 10 minutes, then cut into bite-size chunks.
Make dressing: In a large bowl, whisk together buttermilk, mayonnaise, scallion whites, and garlic; season with salt and pepper.
Add romaine, tomatoes, corn, cheese, scallion greens, and pork to the bowl. Toss to combine, and serve immediately.
To neatly cut up a head of romaine, halve it through the core and make a few evenly spaced cuts lengthwise, leaving core intact. Chop crosswise into bite-size pieces.