Mix the powdered sugar and cocoa together in a small bowl. Set aside.

Scatter half of the granulated sugar on your worktop, place the pastry over the top and roll it out into a rectangle. approximately 10x12 inches.

Brush the pastry with a little of the beaten egg. Then, hold a fine sieve over the pastry and tip in the icing sugar and cocoa mixture. Using the sieve, sprinkle it over the pastry, then spray with water, to help the sugar stick.

With the short end of the pastry facing you, take one long edge in each hand and gently roll the long edges inwards until they meet in the middle. Seal the join with a little beaten egg. Wrap thew rolled pastry in baking paper and then place in the fridge for at least 30 minutes.

Heat your oven to 400 degrees. Remove the pastry roll from the fridge. Cut 1/2 inch-thick slices all along the length and play the slices cut sides up, on the prepared sheets, leaving space for expansion between each slice. Brush the slices with the remaining egg and sprinkle, with the remaining granulated sugar.

Bake the slices for 10-15 minutes, then remove from the oven and turn them over, and cook the other side for about 5-7 minutes, until crisp and golden. Remove from the oven and transfer to a wire rack to cool.