Thaw the puff pastry overnight in the refrigerator or at room temperature for no longer than 40 minutes, flipping it over every 10-15 minutes to help it thaw evenly. Once the dough is pliable, you will want to use it immediately as to prevent the butter inside the dough from melting.
Sprinkle 1/3 cup of sugar onto a large piece of parchment paper. Place the puff pastry on top of the sugar. (Unroll the thawed, store-bought puff pastry.) Sprinkle another 1/3 cup of sugar on top of the puff pastry. Use a rolling pin to roll the dough into a larger square, about 12×12-inches.
Fold the left and right sides inward halfway towards the middle. (I lightly drew a line through the sugar in the center to help eyeball it better.) Fold the left and right sides again to meet in the middle. Then fold the dough in half to make a closed book.
Chill the dough in the fridge for 15-30 minutes to help firm up the butter inside the puff pastry. (TIP: I like to use the parchment paper like a sling to pick up the puff pastry and place it on a sheet pan since you will still need the excess sugar in the next step.)
Preheat the oven to 400 degrees Fahrenheit while the dough is chilling. (Make sure it comes to temperature before the next step.)
Use a sharp knife to cut the dough into 1/2-inch slices. Coat the cut edges with extra sugar and place on a parchment-lined baking sheet. (TIP: I used a ruler and scored the top of the rolled dough to create uniform pieces before cutting through.
Place the cookies 2-inches apart onto parchment lined sheet pans, with no more than 12 per tray. Separate the tips of the cookies like bunny ears to make them bake into a heart shape.
Bake at 400˚F for 12 minutes, then flip them over to the other side and bake 3-7 minutes more, until golden and caramelized. Immediately transfer baked palmiers to a wire cooling rack and wait 10 minutes before eating, as the sugar gets really hot.
Notes
You can bake these for 15-18 minutes straight, without flipping them over. Make sure your oven rack is not too close to the heating element and at least periodically check the bottom of the cookies, if you don’t plan to flip them. Sugar burns shortly after it caramelizes, so don’t walk away from these.
TO STORE: Palmiers are best eaten the day you make them. Store any leftovers in an airtight container at room temperature for up to 5 days.
TO FREEZE: You can only freeze palmiers before baking them. Slice and freeze flat on a tray. Then transfer to a freezer-safe airtight container or bag. Freeze for up to 3 months. Bake for a few extra minutes from frozen.