Mix 500g strong flour, 10g salt, 80g sugar, 10g dried yeast, and 300ml cold milk into a rough dough. Knead briefly — just until combined. Flatten into a rectangle, wrap, and refrigerate overnight.
Place 250g cold unsalted butter between two sheets of baking paper. Beat with a rolling pin into a 20×20cm square. Refrigerate until firm but pliable.
Roll the cold dough into a 40×20cm rectangle. Place the butter in the centre, fold the dough over it like an envelope, and roll out again. Fold into thirds (letter fold). Wrap and chill for 30 minutes. Repeat twice more for 3 total folds.
Roll dough to 5mm thickness. Cut into long triangles with a 10cm base. Gently stretch each triangle and roll from the base toward the tip. Curve the ends inward. Place on lined trays.
Cover loosely and proof at room temperature for 2–3 hours until puffed and wobbly. Brush with egg wash. Bake at 200°C for 15–18 minutes until deep golden.