Preheat the oven to 180 °C/350 °F/Gas Mark 4.
Roll out the pastry on floured greaseproof paper and place a 2cm square block of butter in the middle. Fold each corner of the pastry towards the middle to obtain a square or rectanglular shape.
Dust the pastry and your rolling pin with flour and roll the pastry gently out again, so as to spread the butter evenly within the parcel. Again fold the corners in towards the middle, roll, and repeat the process once more.
With a sharp knife, cut the pastry into triangles 10 x 15 x 15cm. Spread them each with a layer of Apricot Conserve and leave an extra teaspoon-sized dollop in the middle of each. Starting with the flat end of the triangles, carefully roll up the croissants into crescent shapes. Using the pastry brush, glaze them with the egg yolk.
Bake the croissants for 17-20 minutes. Remove from the oven and leave to cool on a wire rack.
Keep in an airtight container and consume within three days.