Unroll your puff pastry on a flat surface and brush with melted butter. Spread generously with a layer of demerara sugar (save a little bit for later), sprinkle with cinnamon powder, and then the white sugar.
Fold the long side of the pastry towards the centre. Same goes for the other side so that the two long edges of the pastry meet at the middle.
Gently press the pastry, or using a rolling pin to gently roll it so that the sugars get planted more firmly with the dough.
Brush with melted butter and sprinkle a little more demerara sugar. Fold again by the long edge so that the dough is now one long log.
Once again, brush with melted butter and sprinkle a little bit of demerara sugar. Flip the log and repeat on the other side.
Using greaseproof paper or clingfilm, wrap the dough and let chill in the fridge for 20-30 minutes.
Preheat oven to 200°C.
Slice up the dough, roughly an inch thick. Place onto a lined baking tray with sufficient space between each Palmier. You're most likely to have to bake in batches. When baking in batches, chill the remaining pastry dough in the fridge until ready to bake.
Bake for 20 minutes until golden and puffed up. Allow to cool down for 5 minutes before transferring onto a cooling rack to let completely cool down.
Notes
The cooking time only includes time for baking one batch.
If using frozen puff pastry, make sure it's thawed before use.
Demerara sugar helps to add a nice crunch to the Palmiers while the white sugar offers a lightly caramelised touch.
You can use any type of sugar of your choice or play around with different ratios to your own liking. Although please do note that different sugars may result in slightly different flavours.
When baking in batches, chill the remaining pastry dough in the fridge until ready to bake.
In my opinion, the Palmiers actually taste better the next day as the sugars crisp up and add a really nice extra crunch to it!