Roll out the puff pastry (mine measured 10x12½ inches / 25x32 cm. Sprinkle the sugar on top and with a rolling pin gently roll over the sugar to press into the dough.
In a small bowl mix the jam with the cornstrarch.
Flip the dough upside down. Spread the jam mixture on top (the plain side). Roll the dough from each end (long side) into the middle making sure they are even and meet in the middle (see photos). Wrap the log in plastic wrap or parchment paper and refrigerate for 30-60 minutes.
Remove the dough from the fridge and cut into ¼-⅓ inch slices, place them cut side up on a parchment paper lined baking sheet about 2 inches / 4 cm apart.
Pre-heat oven to 375F/190C. While the oven is pre-heating refrigerate the palmiers for approximately 15-20 minutes.
Bake for approximately 10 minutes then carefully flip them over and continue to bake for another 5-10 or until golden.
Let cool 10 minutes on the baking sheet, then move to a wire rack to cool completely. Enjoy!
Notes
Storage: Best the day they are baked. Store leftovers in an airtight container at room temperature for up to 2 days, then re-crisp in a 300°F oven for a few minutes. Freeze baked palmiers for up to 1 month, or freeze the unbaked log and thaw overnight in the fridge before slicing and baking.