Thaw spinach and drain. Spread baking dish with oil and arrange on bottom 4 slices of bread. Sprinkle top with 1/2 cup feta cheese, divided evenly spinach (thawed) and sprinkle with half of the feta cheese. Other slices are coated with butter and cover them with stuffing by lining up with buttered side up. Beat with mixer the egg and milk. To this mixture add the basil, salt and pepper. Pour bread banitsa with egg-milk mixture. With a fork to prick the bread a few places to take better topping. Cover the pan with foil and leave in refrigerator for at least half an hour. Then removing fthe oil and bake the banitsa in a hot oven until browned. Remove, wait 5-10 minutes to not quite hot and serve.