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 ⅔ cup (160 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).

Turn out dough onto a lightly floured surface, and roll into a 9-inch square. Wrap in plastic wrap, and refrigerate for 45 minutes to 1 hour.

On a lightly floured surface, roll dough into an 18x10-inch rectangle, lightly flouring 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 spread ¼ cup (50 grams) sugar over center third of rectangle. Fold in thirds like a letter. Repeat procedure for a third and final turn, spreading ¼ cup (50 grams) sugar over center third of rectangle. Wrap dough in plastic wrap, and refrigerate for at least 1 hour. (See PRO TIPS.)

Preheat oven to 350°F (180°C). Spray a 12-cup muffin pan with baking spray with flour. Sprinkle 1 teaspoon (4 grams) sugar in bottom of each cup.

Lightly sprinkle sugar onto wprk surface. Turn out dough onto surface, and roll into a 16x12-inch rectangle. Sprinkle sugar onto dough, lightly pressing with your hands to adhere. Cut 12 (4-inch) squares from dough. Fold corners of 1 square into center. Gather sides toward center, and place in a prepared muffin cup. Repeat with remaining pastry squares. Sprinkle remaining ¼ cup (50 grams) sugar onto pastries.

Bake until golden brown, 30 to 35 minutes. Immediately remove from pan, and let cool completely on a wire rack.

Fill pastries, if desired. Using a small knife, cut a hole in center of each pastry. Spoon preserves and chocolate hazelnut spread into individual pastry bags fitted with a small round piping tip. Pipe into pastries as desired. Serve filled pastries immediately. Store unfilled pastries in an airtight container for up to 3 days.