Preheat oven to 425 degrees. Line a large rimmed baking sheet with parchment paper.

In a small saucepan, add chopped rhubarb, sugar and water for the puree. Bring to a simmer over medium heat and cook for 10 minutes, until the rhubarb is fork-tender. Add the mixture to a blender or food processor and process until smooth. Pass through a fine mesh sieve and cool.

On a clean, lightly floured work surface, place one sheet of puff pastry. Cover the sheet of puff pastry with the cubed butter and top with the other sheet of puff pastry. Roll out the puff pastry sheets into a large, thin rectangle, doubling the original size of the sheets. Be gentle so you do not tear the sheets.

Cut the dough into large triangles from top to bottom. Place 1 tsp of rhubarb puree in the bottom large end of the puff pastry triangles in the center. Top the puree with 1 tsp Nutella. Gently roll up the triangles from the large end to the skinny end, making sure to tuck the tops under the bottom.

Place on baking sheet.

In a small bowl, beat together egg and 1 tsp water. Brush the tops of the croissants with the egg wash.

Bake for 25 to 30 minutes, until golden brown and cooked through.

Let cool and top with powered sugar.