Preheat the oven. Set it to 400°F (200°C). Line a baking sheet with parchment paper to prevent sticking and make cleanup easy.

Prep the puff pastry. Unfold the thawed pastry on a lightly floured surface. If the seams are visible, press them together gently. Use a rolling pin to even it out into a rectangle about 1/8-inch thick.

Cut into squares. Slice the pastry into even squares—about 2 to 2.5 inches each. Aim for 16–20 squares per sheet so they’re bite-size but not fussy.

Add the Nutella. Place a small dollop (about 1 teaspoon) of Nutella in the center of half the squares. Don’t overfill or it will leak during baking.

Seal them up. Lightly brush the edges of the Nutella-topped squares with water. Place a plain square on top, then press the edges together to seal. Crimp with a fork for a tight seal and a pretty edge.

Chill before baking. Transfer the assembled bites to the baking sheet and chill in the fridge for 10 minutes. Cold pastry puffs better and leaks less.

Make the egg wash. Whisk the egg with 1 tablespoon water and a tiny pinch of salt. Brush lightly over the tops for shine and color. Sprinkle with coarse sugar if using.

Vent and bake. Use a sharp knife to cut a tiny slit on top of each bite to release steam. Bake for 14–18 minutes, until puffed and deeply golden.

Finish and cool. Let them cool for 5–10 minutes. Dust with powdered sugar, add a pinch of flaky salt, or scatter chopped hazelnuts while still slightly warm.

Serve warm. They’re best fresh from the oven when the Nutella is silky and the pastry crackles.