Preheat the broiler on high. Place a wire baking rack on a foil-lined baking sheet.
Cut each jalapeño in half and scrape out the seeds and membranes. Place the jalapeños on the prepared baking sheet, and broil until they begin to char slightly and are partially cooked, about 2 minutes. Allow the jalapeños to cool completely.
Preheat oven to 450°F.
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 jalapeño half with the chicken mixture, packing it in tightly.