Preheat the oven to 325 degrees F (165 degrees C).
Generously butter a 9x13 inch baking dish, and coat with some of the bread crumbs. Place the zucchini into the pan, and distribute the cheeses evenly over the top. In medium bowl, mix together the eggs, milk, salt, baking powder and flour. Spoon over the top of the casserole, and dot with butter. Sprinkle remaining bread crumbs over the top.
Bake for 45 minutes in the preheated oven, or until the topping is cooked through. Pick up a piece of the topping and peek underneath to see if it is no longer gooey.