Preheat your oven to 400°F (200°C) and line a baking sheet with parchment paper.
Sprinkle a clean surface with a generous amount of granulated sugar.
Place one sheet of puff pastry on top of the sugar and roll it out into a rectangle, about 1/8 inch thick.
Brush the surface of the puff pastry with melted butter, making sure to cover it evenly.
In a small bowl, mix together the granulated sugar and ground cinnamon (if using).
Sprinkle the sugar mixture evenly over the buttered puff pastry, pressing it gently with your hands to make it adhere.
Starting from one of the longer edges, tightly roll the puff pastry into a log shape.
Repeat the process with the second sheet of puff pastry.
Using a sharp knife, slice the rolled puff pastry into 1/2-inch thick slices.
Place the slices on the prepared baking sheet, leaving some space between them as they will expand during baking.
Gently press down on each slice with the palm of your hand to flatten it slightly.
Bake the French Palmiers in the preheated oven for 12-15 minutes, or until they are golden brown and crispy.
Remove the baking sheet from the oven and let the palmiers cool completely before serving.