Preheat oven to 400°F (204°C). Line a large baking sheet with parchment paper.
Place 1 puff pastry sheet on the baking sheet. Spread a thin layer of Nutella on top.
Place the second puff pastry sheet on top. Press lightly to seal.
Using a sharp knife or pizza cutter, cut the pastry into 8 equal strips.
Shape each strip into a heart. Twist the strip once, curve the top into a heart shape, and pinch the bottom point to hold it!
Optional: Slide a soaked wooden skewer through each heart before baking.
Whisk the egg. Brush a light coat of egg wash over each heart.
Bake for 12–15 minutes, until puffed and deeply golden.
Cool for 5 minutes. Dust with powdered sugar.
If using strawberries, add them after baking by sliding onto the ends of the skewers. Serve warm.