Preheat oven to 375 degrees. Melt 2 T butter in the 8 x 11.5 casserole dish in the oven while the oven is preheating.
While butter is melting, whisk together the remaining ingredients: flour, sugar, milk, egg, and vanilla until smooth. Pour over melted butter and bake 20 minutes or until the center is set.
You can insert a toothpick to test for doneness. When stuck in the middle of the cake and it comes out clean the pancake is done. This pancake will not brown on the top so do not use that as an indication of doneness. Remove from oven and place pats of butter on top if desired. Cut with a sharp knife in large squares and serve with maple syrup.