Preheat oven to 400˚F.
Spread the melted butter over the bottom of a 9x13 casserole dish. Place the chicken and frozen vegetables evenly over the butter.
Whisk together the biscuit mix and 2 cups of milk. There may still be some lumps, that is okay. Pour the mixture over the chicken and veggies. Do not mix.
Whisk together the cream of chicken soup and chicken broth. Pour mixture over the casserole. Do not mix.
Bake for 40-50 minutes or until the sides are bubbling and the top starts to brown. Let sit 5 minutes to set, then serve.