Put the butter and water in a pan with the leek, cover with a tight-fitting lid and cook over medium heat for 8 to 10 minutes until tender. Leave to cool, then mix in the cheese and ham.
On a lightly floured surface, roll out the pastry to the thickness of about 3mm. The using a 10cm round cutter, cut out as many discs as you can - generally, about 8-10.
Place a heaped teaspoon of the leek, ham and cheese mixture in the centre of each disc of pastry. Brush the edges with the beaten egg, fold the discs in hald and crimp well with your fingers or the tines of a fork dipped in flour to seal. Brush the puffs with the remaining beaten egg, place on a tray and freeze.
Cook from frozen for 20 to 25 minutes, until golden brown, in an oven preheated to 190c/375f. When done, remove from the oven and allow to cool before serving.