Heat the oven to 400°F (200°C), or 375°F (190°C) for a fan or convection oven. Line a baking sheet with parchment paper so the pastry trees release easily after baking.

Place one thawed puff-pastry sheet on a lightly prepared work surface. Spread the Nutella evenly across the pastry, leaving a small border around the edges so the filling stays contained.

Lay the second puff-pastry sheet directly on top of the chocolate layer. Press gently across the surface to help the two pastry sheets adhere without flattening the layers.

Cut the layered pastry into individual triangular tree shapes, making each long triangle about 2 to 3 inches wide at the base. Depending on the size of the cuts, you should have approximately 8 to 10 trees.

Make horizontal cuts into both sides of each triangle, moving toward the center while leaving the middle strip intact. Twist each cut section once or twice to create branch-like shapes, taking care not to pull the pastry apart.

Transfer the shaped trees to the parchment-lined baking sheet, leaving a little space between them. Insert a small wooden skewer or lollipop stick into the base of each tree if you want to serve them as pastry pops.

Beat the egg with the water in a small bowl. Brush a thin, even coating over the exposed pastry, avoiding pools of egg wash in the cuts and around the edges.

Chill the shaped pastry for about 10 minutes if it has become soft, sticky, or difficult to handle. Bake for 18 to 22 minutes, until the trees are puffed and deeply golden.

Let the pastries cool briefly on the baking sheet before moving them to a serving platter. Dust with confectioners’ sugar and add sprinkles or pearl sugar while they are still slightly warm.