Spray a 9x13-inch baking pan with nonstick spray.
Preheat the oven to 375°F.
Place the biscuits on a baking sheet and bake for 8 minutes, then remove and keep the oven on.
In a large skillet over medium-high heat, melt 2 tablespoons of butter.
Add the frozen vegetables and cook for 10 to 15 minutes until tender.
Stir in the cooked chicken, milk, cream of chicken soup, salt, and pepper; cook for 5 minutes until heated through.
Pour the mixture into the prepared baking dish and spread evenly.
Arrange the pre-baked biscuits on top of the casserole, baked side down and raw side up.
Melt the remaining 2 tablespoons of butter with garlic powder in a covered microwave-safe dish, microwaving in 30-second increments until melted.
Drizzle the melted butter over the biscuits.
Bake the casserole in the oven for 10 to 12 minutes, then tent with foil and bake for another 10 to 12 minutes until biscuits are golden brown.