Combine flour, cornmeal and salt in medium-size bowl. With pastry blender or 2 knives held scissor fashion, cut in butter until mixture resembles coarse crumbs. Sprinkle cold water, 1 tablespoon at a time, over mixture, tossing lightly with fork after each addition until pastry is moist enough to hold together. Shape pastry into ball. Cover with plastic wrap. (Pastry can be prepared up to 3 days ahead and refrigerated.)
Heat butter in large skillet over medium-high heat. Add green onion, carrot and chili powder; cook, stirring occasionally, until carrot is slightly softened, about 5 minutes. Stir in flour; cook, stirring constantly, 1 minute or until smooth and bubbly. Stir in milk and chicken broth; cook, stirring constantly, until mixture thickens, 1 to 2 minutes. Stir in cooked chicken, black beans, corn kernels, green chilies if using and salt. Spoon chicken mixture into 13 x 9 x 2-inch baking dish. (Filling can be prepared up to a day ahead and refrigerated, covered.)
Roll out the pastry on a lightly floured surface with a floured rolling pin into rectangle 1 inch larger than baking dish. Center pastry over filling. Fold overhang back over top of pastry; press to form decorative edge. With sharp knife, cut decorative vents in pastry. (Whole pie can be assembled several hours ahead and refrigerated. Add extra time for baking.)
To bake, heat oven to 350 degrees F. Bake pie 35 to 40 minutes, until pastry is golden and filling is bubbly. Makes 6 servings.