Make dough: mix flour, sugar, salt, yeast (or use prepared sourdough starter), warm water or milk, and butter; knead until smooth; cover and rest until doubled
Prepare butter block: roll cold butter into a flat rectangle about the size of the dough, keep chilled
Laminate: roll dough into a rectangle, place butter block on it, fold dough over butter; seal edges
Roll and fold: roll into a longer rectangle without breaking layers; fold into thirds (letter fold) or into quarters (book fold); chill 20–30 minutes
Repeat folds: perform 3–6 total turns (each turn = roll + fold + chill), keeping butter cold and dough relaxed
Final chill: chill dough 1–2 hours (or overnight) after the last turn
Shape: roll dough to about 3–4 mm thick; trim edges; cut into triangles
Proof: roll triangles from wide end to narrow end; place on baking tray with tip underneath; cover and proof until puffy (about 1–2 hours depending on temperature)
Egg wash: brush with egg wash (egg + a splash of water or milk)
Bake: bake at 200–220°C (390–425°F) until deep golden brown (about 15–25 minutes)
Cool: cool on a rack 10–20 minutes before serving