Important: Thaw the puff pastry at room temperature just until it becomes tacky. Don’t leave it out for too long, or it will become elastic and difficult to work with. Keep the paper or plastic on the pastry, as you’ll use this to help flip it later.
Mix the sugars and salt together in a bowl until well combined. Sprinkle half of the sugar evenly over one side of the pastry, making sure it’s fully covered. Use a rolling pin to gently press the sugar into the pastry. Using the paper or plastic, flip the pastry over, then peel it off and repeat the process with the remaining sugar.
Fold the long sides of the pastry inwards so they meet in the centre, then fold them in again. Fold one half over the other like a book to form a compact log with a heart-shaped centre. See my video in the recipe card below for guidance. Chill the pastry in the fridge, uncovered, for 30 minutes until firm.
Preheat the oven to 200°C (390°F). Line 1–2 baking trays with non-stick baking (parchment) paper. Slice the chilled pastry into 1 cm (0.4 in) pieces — you should get around 15 slices. Arrange the slices on the prepared trays, leaving a space between each to allow for spreading.
Bake for 13–15 minutes, until the palmiers are firm enough to flip. Using a spatula, carefully turn each pastry over and bake for a further 5–8 minutes, or until crisp and golden. Transfer the palmiers to a cooling rack and allow them to cool completely before serving.