Defrost the Puff Pastry - Defrost overnight in the refrigerator or leave it in its package on the counter at room temperature for about 40 minutes. To thaw in the microwave, remove 1 pastry sheet from the packaging, wrap in a paper towel, and microwave on high for 15 seconds per side.
Mix the Sugar and Cinnamon - Combine the sugar and cinnamon in a medium bowl.
Roll Out the Puff Pastry - Unfold 1 defrosted pastry sheet on a piece of parchment paper. The puff pastry should be refrigerator temperature, and completely defrosted. With a rolling pin, roll out the puff pastry sheet in both directions to create a slightly flatter and even surface.
Add the Cinnamon Sugar Mixture - Evenly cover the surface of the puff pastry with ¼ cup of the cinnamon-sugar mixture. Using the rolling pin, roll over the sugar surface; this helps the mixture adhere to the puff pastry.Carefully flip the pastry over (the excess sugar mixture will come off), and repeat the step on the other side, gently rolling the sugar into the surface of the dough. You want an even covering of the sugar mixture on both sides of the pastry. Roll the dough out to approximately a 13” by 13” square.
Fold the Pastry - To shape the palmiers, fold the dough evenly from the opposite edges inward toward the center twice, then fold one half over the other to form a closed book shape with six layers. Repeat with the second sheet of puff pastry.
Refrigerate - Chill the dough for 15 minutes to allow the fat in the puff pastry to harden slightly before cutting and prevent melting into the dough.
Heat the Oven - Set the oven to the center position. Heat the oven to 400°F (204ºC).
Slice - Slice the dough into ⅜ inch slices. This should yield approximately 24 cookies per puff pastry sheet.
Coat - Dip each side of the sliced dough into the cinnamon sugar mixture. Place the cookies cut-side up, 2" apart on baking sheets lined with parchment paper. For a more heart-shaped cookie, slightly separate the tops of the cookie dough to resemble bunny ears.
Bake - Bake one tray at a time in the center of the oven. Bake for 12 minutes, then turn over the cookies with a spatula and bake another 3 minutes, or until lightly caramelized on the surface. This may require 4 to 7 minutes depending on your oven. Check after each additional minute because caramelization will occur quickly. The cookies are done baking when both sides feel hard on the surface.
Cool - Immediately transfer the cookies to a wire rack. They will continue to harden as they cool. The texture should be crunchy and crispy.
Notes
Storing: Store the cookies in an airtight container for up to 5 days.