Sift the ﬂour and salt into a large mixing bowl and make a well in the centre. Beat in the egg and a little of the milk until smooth, then gradually beat in the remaining milk and 4 tablespoons of the melted butter. Cover the bowl and leave to stand for 1 hour.
Preheat the oven to 140°C.
Stir the batter and pour it into a measuring jug. Heat about 1 teaspoon of the remaining butter in a small frying pan until sizzling, taking care not to let it darken or burn.
Pour one-twelfth of the batter (about ¼ cup) into the pan, turning the pan to coat the base evenly. Cook over medium heat until the pancake is light golden brown underneath and appears dry on top. Flip over and cook the other side until golden. Turn the pancake out on a plate and sprinkle it with a little of the lemon juice and sugar, then roll it up or fold it in halves twice.
Transfer to a large heatproof serving plate and cover with greaseproof paper before putting it into the oven to keep warm. Make 11 more pancakes in the same way.
Sprinkle the ﬁnished pancakes with any remaining melted butter, lemon juice and sugar. Decorate with the lemon slices and serve immediately.