This recipe is adapted from William Curley and Justin Gellatly. The puff pastry makes more than you need, but it freezes really well and you can use it for other recipes.
To make your butter block, leave the butter at room temperature, then coat the butter and your hands in flour, knead the butter until it softens and then add shape it into the square 15cm by 15cm. Wrap the butter in clingfilm and return it to the fridge to firm up.