Sift the flour into a large bowl, add the butter and rub into the flour with your fingertips until the mixture resembles fine breadcrumbs. Make a well, add 3 tablespoons water and mix with a flat-bladed knife, using a cutting action, until the mixture comes together in beads. Gather the dough together and lift out on to a lightly floured surface. Press into a ball and flatten into a disc. Wrap in cling film and chill for 15 minutes.
Preheat the oven to 200°C/400°F/Gas mark 6. Grease two baking trays with butter.
Heat the oil in a large frying pan, and sauté the ginger, garlic and prawns for 3 minutes. Remove the prawns and set aside. Add the chilli sauce, lime juice and cream to the pan, and simmer over a medium heat until the sauce has reduced by a third. Return the prawns to the pan and add the coriander. Stir through, then leave to cool.
Divide the pastry into four, and roll out each portion, between sheets of baking parchment, into a 20cm/8in circle. Divide the filling into four, and place a portion in the centre of each pastry, leaving a wide border. Fold the edges loosely over the filling, and slide the pies onto the prepared baking trays. Brush the pastry with the egg yolk. Bake for 25 minutes or until golden. Serve hot.