Put flour, salt, xylitol and yeast in a large mixing bowl. Slowly pour in the cold water and bring together in a stand mixer or with your hands until it forms a stiff dough. Knead in the mixer or by hand for about 8-10 mins. Shape into a ball, cover with cling film or place in a freeze bag and chill in the fridge for at least 1 hour.

Place the cold butter between two sheets of baking/greaseproof paper and bash or roll it out flat with a rolling pin. You want it to be evenly rolled to a rectangle shape about 40x20cm (doesn't have to be exact). Still wrapped in the greaseproof paper, chill the butter again to make sure it is still really cold.

Roll out the chilled dough to make a 60x21cm rectangle. You basically want it to be a third taller than the butter one, and a little bit wider (again, doesn't have to be exact as long as it is roughly in proportion with the butter).

Place the chilled butter rectangle on top of the bottom 2/3 of the rolled out dough, then fold over the third of the dough without butter on it over the top third of the 'buttered' bit. Gently score along the top of the bit of butter that is still exposed so it can fold over well (or cut it off and put it on top of the folded dough). Fold the bottom bit of dough over so you have folded the whole thing like a business letter. Push lightly on the edges of the dough to seal, then wrap in cling film and chill again for about 1 hour.

Now you are ready to shape the croissants: Roll out the dough one more time, then cut in half lengthways. Cut each strip into 6 triangle shapes by cutting into 3 square shapes then cutting diagonally across each square. You should now have 12 triangles of dough. Roll each triangle up from the thick end through to the point, so you have a spiral "croissant" shape. Turn the ends of the croissant inwards so they are almost touching if you want a more traditional shape. Repeat with the rest of the triangles, placing each croissant on a lined baking tray (you may need two), leaving a decent amount of space between each croissant. Cover the shaped croissants on the trays with cling film and leave to rise for another 1-2 hours.

Preheat the oven to 200C/400F/gas 6. Beat the egg with a splash of milk or water and brush the croissants with it. Bake in the preheated oven for 20-25 mins, until the croissants are risen, golden and cooked through. Allow to cool slightly before eating. Best eaten fresh, but you can easily freeze the croissants before baking when you have shaped them - just pop the tray of shaped pastries in the freezer for a couple of hours, then place the frozen croissants in a freezer bag or freezer-safe container. When you are ready for them, just brush with egg and bake from frozen for an extra 5 mins. Yummy served with sugar free jam, low sugar chocolate spread, unsweetened nut butters, or savoury toppings. Or just eat warm on their own dipped in a cup of coffee or hot milk. YUM.