Prepare a baking tray with parchment paper. Set aside.
On a large work surface, sprinkle a generous amount of granulated sugar. Place the puff pastry directly on top of the sugar & sprinkle another generous layer on top of that. Using a chilled rolling pin, roll the puff pastry out into a rectangle about 1/4-inch thick, around 12-13inches long. The puff pastry should be studded with sugar granules.
Use a pastry brush to cover the puff with an even layer of melted butter.
Liberally douse the melted butter with a generous layer of vanilla sugar.
Going from short-side to short-side, gently fold the dough towards the middle three times. Repeat on the other short-side. The two folded sides should meet in the middle, with a small gap between them. Fold one side over the other, as if closing a book. Lightly press the sides together. Transfer the log of dough to the prepared baking sheet & place in the fridge for 20-minutes.
Preheat the oven to 200°C.
Using a sharp knife, cut the dough into 10 equal pieces. Place the palmiers filling side up on the prepared baking sheet, leaving space between each. I like to gently squish the palmiers down with my fingers to make sure all those layers of sugar and butter are truly stuck together.
Generously sprinkle the squished palmiers with more vanilla sugar, and bake in the preheated oven for 25-minutes, flipping each palmier over halfway through. The finished palmiers should be deep golden brown and lightly caramelized.
Transfer to a wire rack to cool completely. The palmiers will become flaky and delicious as they cool, so try to resist the temptation to eat when warm. Enjoy!