Preheat your oven to 375°F. Line 2 large rimmed baking sheets with unbleached parchment paper, and set aside.

Prepare the gluten free puff pastry according to the recipe instructions.

The puff pastry can be made up to 3 months ahead of time and frozen, then defrosted overnight in the refrigerator. It can also be made up to 4 days ahead of time and refrigerated until ready to use.

Mix the sugar and salt in a separate, medium-size bowl, until well-combined. Sprinkle 1/3 cup (67 g) of the sugar on a large, flat surface such as a large pastry board, cutting board or smooth countertop, and spread into an even, rectangular layer (about 14-inches x 16-inches in area).

Place the chilled puff pastry dough on top of the sugar, sprinkle lightly with flour, and roll into a rectangle that is about 14-inches x 16-inches, and a bit more than 1/8-inch thick. Trim any irregular edges of the rectangle with a pastry cutter, pizza wheel or very sharp knife.

Brush the entire top of the puff pastry rectangle with the egg white in a thin, even layer.

Sprinkle another 1/3 cup (67 g) of the sugar on top of the puff pastry rectangle and spread into an even layer, and press down gently to help the sugar adhere to the dough.

Gently fold each 16-inch side of the pastry rectangle 1/4 of the way toward the center.

Fold the same sides again, another 1/4 of the way toward the center so that they meet in the middle.

Finally, fold both edges of the now 8-inch rectangle along the center seam over one another, creating a 4-inch by 14-inch rectangle with 6 distinct layers of dough. Place the entire rectangle on the prepared baking sheet, and then in the freezer for 10 minutes or until firm.

Once the dough is firm, remove from the freezer and transfer the dough to a cutting board.

Using a very sharp knife, slice the dough by cross-section into 24 equal pieces, each about 1/2-inch wide.

Dip both sides of each piece in the remaining 1/3 cup (67 g) of sugar, and then place on the prepared baking sheets, about 2 inches apart from one another.

Press the flat bottom of a glass firmly and evenly onto the top of each of the cookies on the baking sheet to compress the layers by about 1/4.

Place in the freezer for about 5 minutes, or until once again firm.

One at a time, place the baking sheets in the preheated oven for 9 minutes, or until the cookies are lightly golden brown on the underside.

Working quickly, remove the baking sheet from the oven, and carefully flip each cookie over.

Return to the oven and bake for another 3 to 4 minutes, or until lightly golden brown on the second side.

Remove from the oven and allow to cool on the baking sheet for 5 minutes before transferring to a wire rack to cool completely.

Cookies will stay fresh for about 3 days if stored in a sealed glass container at room temperature. For longer storage, seal well in a freezer-safe container and freeze.