Preheat your oven to 425 degrees. Roll out or unfold your thawed puff pastry sheets. Cut into 4 even squares each.
Place 2 pieces of chocolate close to a corner of a puff pastry square and roll the corner up, continuing to roll the pastry dough and seal the opposite corner over. Working with the left side first, roll over the edges of the dough to close. Repeat on right side. Then, join together the two ends in the middle. Press gently to secure while baking. Repeat with remaining squares.
When ready to bake, brush on the egg wash over each croissant, sprinkle on the sliced almonds and bake for about 15-17 minutes or until the croissants have reached a nice golden brown color.
When finished, allow to cool before drizzling on the melted Nutella. Serve immediately.