Preheat the broiler on high. Place a wire baking rack on a foil-lined baking sheet.
Cut each jalapeno in half and scrape out the seeds and membranes. Place the jalapenos on the prepared baking sheet, and broil until they begin to char slightly and are partially cooked, about 2 minutes. Allow the jalapenos to cool completely.
Preheat oven to 450F.
In a small bowl, mix together the chicken, cream cheese, sour cream, scallions and cheese. Season with salt and pepper to taste. Divide the filling into 12 equal parts and fill each jalapeno half with the chicken mixture, packing it in tightly.
Put the flour in a shallow dish. Put the panko in a small dish. In a medium bowl, whip the egg whites with a whisk until they are extremely foamy but not quite holding peaks.
Working in batches, dredge the jalapenos in the flour, shaking off any excess. Add the jalapenos to the egg whites and toss to coat completely, bring careful not to let the filling come out. Add the jalapenos, a few pieces at a time, to the panko and coat completely.
Spread the jalapenos out on the wire rack and season them generously with salt and pepper. Spray the jalapenos lightly with cooking spray. Bake until the breading is golden brown and crispy and the cheese is melted throughout, about 20 minutes.