1. Make the crust: In a food processor, pulse together flour, sugar, and salt. Add butter and pulse until mixture resembles coarse meal, with a few pea-size pieces of butter remaining. Sprinkle with 3 tablespoons ice water. Pulse until dough is crumbly but holds together when squeezed (if necessary, add up to 2 tablespoons ice water); do not overmix. Form dough into a disk, wrap tightly in plastic, and refrigerate until firm, 1 hour or overnight (or freeze, up to 1 month).

2. Preheat oven to 375 degrees. Make the filling: In a large pot, melt butter over medium-high. Add onion and tomatoes and cook until softened, 8 minutes. Add garlic and chili powder and cook until fragrant, 30 seconds. Add flour and stir to coat vegetables. Slowly add broth, whisking constantly until sauce is smooth. Bring to a boil, reduce heat, and simmer until thickened, 5 to 7 minutes. Stir in corn. Season with salt and pepper, then stir in chicken. Pour filling into a 2-quart baking dish.

3. On a floured work surface, roll out dough to 1/8-inch thickness. Place dough over dish and fold overhang inward while pinching to crimp edge. Cut vents in dough. Place dish on a rimmed baking sheet and bake until crust is golden brown and filling is bubbling around edge, 45 to 50 minutes. Let cool 15 minutes before serving.