In a heavy 5-quart pot, melt the butter over medium-high heat. Add the poblano, onion, celery, and garlic and cook, stirring occasionally, until softened but not browned, about five minutes.
Add flour and stir until vegetables are evenl coated.
Add broth, potatoes, chili powder, salt, and pepper.
Raise heat to high and bring to a boil. Reduce heat to maintain a simmer, cover, and cook until potatoes are tender, about 20 minutes.
Add corn and simmer until warmed through.
Off the heat, stir in half and half, cotija, lime juice, scallions, and cilantro.