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 tsp salt and ¼ tsp pepper. Place pork on prepared baking dish; rub with spice mixture. Broil pork 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, cheese, scallion greens, and pork to the bowl. Toss to combine and serve immediately.