In an 8-inch square baking pan, melt the butter, coating sides of pan and leaving remaining butter in the bottom of the pan. Set aside.
In large bowl, beat eggs till frothy, then add milk and stir.
In separate bowl, combine flour, salt, pepper and baking powder. Drain spinach well, then add to dry ingredients and mix thoroughly.
Add this mixture to beaten eggs and milk, then add grated cheese, oregano and cayenne (if using).
Pour all into buttered pan and bake at 350 degrees for 30 minutes. Remove from oven and let rest for 10 minutes. (This may appear slightly wet before resting, but is done if firm to the touch.) Cut into four equal portions and serve while still warm.