Preheat the oven to Gas Mark 6, 200°C, fan 180°C. Melt the butter in a large pan over a gentle heat. Add the leeks with a little seasoning and cook for 3 minutes or until softened. Remove from the heat and set aside to cool.
In a large bowl, mix the chicken, cooled leeks, crème fraîche, mustard and tarragon until well combined. Season the mixture and give everything another stir.
Unroll one pack of pastry and stamp out 12 discs using an 8cm (3 1/2in) pastry cutter. Place the cut discs on a baking tray lined with nonstick baking paper and chill while you prepare the other rounds.
Unroll the remaining pastry and stamp out another 12 rounds, this time using a 9cm (4in) pastry cutter (if necessary, roll out the pastry a little with a rolling pin to mzake it larger). Transfer to a baking tray lined with nonstick paper and chill for 10 minutes.
Meanwhile, remove the 8cm rounds from the fridge. Pile a heaped tablespoon of the filling mixture in the middle of each round, leaving a 1cm (1/2in) border. Brush the borders with the beaten yolk, then place the 9cm (4in) rounds over the filling and press down firmly to seal the edges, ensuring there is no air trapped. Crimp the edges with a fork, then chill for 10-15 minutes to firm up.
Brush the pies with the remaining yolk, then poke a small air hole in the centre of each. Bake for 20 minutes until puffed and golden. Remove from the oven and leave to cool on a wire rack, before transporting to your picnic.
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For top tips on protecting you and your family when preparing raw meat and poultry, visit Food Safety in the Home.