Preheat the oven to 400 degrees and spread melted butter in a 9x13 baking pan. Layer shredded chicken and frozen vegetables over the butter. In a bowl, mix biscuit mix and milk, then pour it over the chicken and vegetables without stirring.
Whisk together chicken soup and broth, pour over the biscuit layer without mixing. Bake for about 45 minutes until golden brown. Allow it to sit for 5-10 minutes before serving.
For extra flavor, melt 4 tablespoons of butter, mix with the seasoning packet from the biscuit mix, and brush over the casserole while resting (optional).