Heat a 20cm non-stick pan and brush with melted butter. Stir the batter and add more milk if it has thickened. Pour 60-70ml of the batter into the pan and swirl so the mixture covers the base. Cook for 1 minute, loosen around the edge with a spatula, then turn the crêpe over and cook for a further minute. Slide the crêpe out of the pan, then repeat, greasing the pan with butter in between.