Preheat the Oven: Preheat the oven to 375°F (190°C).
Prepare the Pastry: Unfold the puff pastry sheet and place it on a baking sheet lined with parchment paper or a silicone baking mat.
Spread the Hazelnut Filling: Use an offset spatula or spoon to spread the chocolate hazelnut spread evenly over the middle third of the pastry sheet.
Slice the Banana: Cut the banana into ¼-inch slices.
Add the Banana: Arrange the banana slices in a layer over the chocolate hazelnut spread.
Cut the Pastry: Using a knife, make 1-inch diagonal slits along both sides of the pastry next to the filled center section.
Trim the Corners: Cut away the four corner triangles of puff pastry and discard them.
Form the Braid: Starting alternately from each side, fold the diagonal pastry strips over the filling, crossing them over one another to create a braided appearance.
Brush the Pastry: Lightly brush the surface of the completed braid with the lightly whipped egg. If desired, sprinkle the top with raw sugar or granulated sugar. Bake the Braid: Bake for 15–18 minutes, or until the puff pastry is golden and cooked through.
Cool Before Serving: Remove the pastry from the oven and let it cool for 10–15 minutes before serving. For the best texture and freshness, serve the pastry on the day it is made.