1. In the bowl of a stand mixer fitted with the paddle attachment, beat flour, cold butter, and salt at low speed just until butter is coated with flour. With mixer on low speed, add ½ cup (120 grams) ice water in a slow, steady stream, beating just until dough comes together, about 1 minute, stopping to scrape bottom and sides of bowl and turn dough to hydrate evenly. (There will still be large pieces of butter. It is OK if a few dry bits remain.)

2. Turn out dough onto a lightly floured surface, and roll into a 7-inch square. Wrap in plastic wrap, and refrigerate for 45 minutes to 1 hour.

3. On a lightly floured surface, roll dough into an 18x10-inch rectangle, lightly flouring work surface and top of dough as needed. Fold dough in thirds like a letter. Rotate dough 90 degrees; roll into an 18x10-inch rectangle, and fold in thirds like a letter. Repeat procedure for a third and final turn. (If at any point the butter is too soft after a fold, wrap in plastic wrap and freeze until butter is firm again, checking every 5 minutes). Wrap dough in plastic wrap, and refrigerate for at least 1 hour or up to overnight.

4. Line 2 baking sheets with parchment paper.

5. Divide dough in half. On a lightly floured surface, roll each portion into a 13-inch square. Spread hazelnut chocolate spread onto 1 square, leaving a ½-inch border around all sides. Place remaining dough on spread. Freeze stacked dough until firm, about 10 minutes.

6. Using a sharp knife, pastry wheel, or pizza cutter, trim edges of dough to straighten sides; cut dough into 12 to 13 strips (about 1 inch wide). Gently twist each dough strip 4 to 5 times. Transfer to prepared pans. Cover with plastic wrap, and freeze until firm, about 20 minutes.

7. Preheat oven to 400°F (200°C).

8. Brush egg onto pastry.

9. Bake until golden brown, 15 to 20 minutes. Let cool on pans for 5 minutes. Remove from pans, and let cool completely on wire racks. Store in an airtight container for up to 2 days.