In a large bowl, whisk together the flour, cinnamon, sugar and ½ tsp salt. In a jug, whisk the egg, milk, vanilla and melted butter. Make a well in the dry ingredients and pour in the milk mixture gradually, whisking as you go to make a smooth batter. Ideally, let the mixture stand for 1 hr (or even overnight, covered in the fridge), although you can cook with it straight away.