Baked Potato Chicken and Broccoli Casserole

4 large russet potatoes, peeled and cubed
2 cups cooked and diced chicken breast
1 head of broccoli, cut into florets
1 cup sour cream
1/2 cup milk
2 cups shredded cheddar cheese
1 teaspoon garlic powder
1 teaspoon onion powder
Salt and pepper to taste
1/2 cup breadcrumbs
2 tablespoons melted butter