Thaw the puff pastry sheets (if using frozen) but make sure they are still cold.
Line 2 baking pans with parchment paper.
Unfold one of the puff pastry sheets and cut it into 6 rectangles, as in the process image.
Arrange half of the chocolate chips on the 6 pastry rectangles.
From the short end, roll up each rectangle. Repeat with the rest and place all 6 onto your prepared baking pan. Whisk the egg with a splash of water or milk and apply a light egg wash.
Then, continue the same with the next puff pastry sheet. Overall you will get 12 chocolate puff pastries. Bake 6 at once.
If at any point the puff pastry becomes warm to handle, pop it into the freezer or fridge for a few minutes to chill. Do not force the pastry if it is too warm.
Pre-heat the oven to 200C / 392F (no fan).
Chill the pastry for 30 minutes then bake them at 200C / 392F (no fan) for about 20 minutes. If the pastries were browning too much, cover them with aluminum foil.
Let them slightly cool in the baking tray for 3 minutes then move them onto a cooling rack.
The chocolate puff pastries are the best served fresh. Any leftovers can be stored at room temperature for 2-3 days in an airtight container.
Notes
Work as quickly with the pastry as you can. Over time as the pastry warms, it will stick and becomes very difficult to handle.
Check the bottom of the pastry to see whether it is well baked.