Preheat oven to 425°F (220°C).
In a saucepan, melt butter over medium heat. Add carrots, peas, and celery; cook until tender.
Stir in flour, salt, pepper, and garlic powder until well blended.
Gradually stir in chicken broth and milk. Cook and stir until thickened and bubbly.
Remove from heat and stir in cooked chicken.
Place one pie crust in a 9-inch pie plate. Pour in the chicken mixture.
Cover with second crust, seal edges, and cut slits in the top to vent.
Bake for 30-35 minutes or until crust is golden brown and filling is bubbly.
Let stand 10 minutes before serving.