Thaw frozen puff pastry overnight in the fridge, or on the counter for 30 minutes. (I keep my frozen puff pastry sheet in its packaging and transfer it to the fridge to thaw overnight.)
In a bowl, whisk together sugars, salt, and optional pumpkin pie spice or cinnamon.
Spread ⅓ cup of the sugar mixture over a flat, clean work surface, then place the thawed puff pastry sheet on top.
Add another ⅓ cup of the sugar mixture on top of the puff pastry sheet.
Roll out the puff pastry dough into a square (around 13x13-inches) or a rectangle, until the dough is less than ¼-inch thick. As you roll it out, press the sugar into the dough with the rolling pin.
Lightly score a line (crosswise) to mark the middle of the puff pastry. (I used a ruler to make it easy.)
Starting with the shorter side of the pastry sheet, roll up the dough “jelly-roll” style, up to the middle line. Repeat on the other side, so the two rolls meet in the middle.
Transfer rolled dough log to a baking sheet and freeze for 30 minutes to firm up.
Preheat oven to 425°F (220°C).
Once firm, cut dough log into slices around ½-inch thick or less. (I aim for ⅜-inch).
Transfer to a parchment-lined baking sheet, leaving space between for them to puff up and spread in the oven. Sprinkle with remaining sugar mixture.
Bake 10-12 minutes. (No need to flip.)
Cool on baking sheet for 5 minutes and then transfer to a wire rack to cool completely. They will crisp more as they cool.
Enjoy immediately, or store in an airtight container for a few days.
Best enjoyed fresh on the same day, but you can store them in an airtight container for a few days.
Yields 18 palmier cookies.