Melt half the butter in a frying pan and cook the leeks for 5–6 minutes, until soft and golden. Add the mushrooms, chestnuts and tarragon and cook for a further 4–5 minutes, until softened. Pour in the cider and simmer until completely evaporated. Stir in the cream, then season and simmer for 1 minute to thicken slightly. Take off the heat and keep warm. Meanwhile, sift the flour into a bowl with a pinch of salt. Make a well in the centre and pour in the milk and egg. Mix well, using a balloon whisk, until the batter is smooth.
Melt a knob of the remaining butter in a nonstick frying pan or 24 cm (9½ inch) crêpe pan over a medium heat. Pour a quarter of the batter into the pan and swirl to spread out. Cook for 1–2 minutes, until set and lightly golden underneath. Flip and cook the other side for 30–60 seconds, until golden. Transfer to a plate and keep warm. Repeat to make another 3 pancakes with the remaining batter.
Place the pancakes on warmed plates, fill with the leek and chestnut mixture, then fold and serve with a mixed salad.