Combine the sugar and kosher salt. Pour 1 cup of the sugar/salt mixture on a flat surface such as wooden board or marble. Unfold each sheet of puff pastry onto the sugar and pour 1/2 cup of the sugar mixture on top, spreading it evenly on the puff pastry. This is not about sprinkling, it's about an even covering of sugar. With a rolling pin, roll the dough until it's 13 by 13-inches square and the sugar is pressed into the puff pastry on top and bottom. Fold the sides of the square towards the center so they go halfway to the middle. Fold them again so the two folds meet exactly at the middle of the dough. Then fold 1 half over the other half as though closing a book. You will have 6 layers. Slice the dough into 3/8-inch slices and place the slices, cut side up, on baking sheets lined with parchment paper. Place the second sheet of pastry on the sugared board, sprinkle with the remaining 1/2 cup of sugar mixture, and continue as above. (There will be quite a bit of sugar left over on the board.) Slice and arrange on baking sheets lined with parchment.
Bake the cookies for 6 minutes until caramelized and brown on the bottom, then turn with a spatula and bake another 3 to 5 minutes, until caramelized on the other side. Transfer to a baking rack to cool.