Make the dough (détrempe): Mix flour, sugar, salt, and yeast in a large bowl. Add warm milk and knead until smooth (about 10 minutes). Cover and rest 1 hour until doubled. Punch down, wrap, and refrigerate at least 30 minutes.
Prepare the butter block (beurrage): Place butter between parchment paper and pound/roll into a 6x8 inch rectangle. Chill until firm but pliable.
Laminate: Roll dough into a 12x8 inch rectangle, place butter in center, fold like a letter. Roll out, fold into thirds (one turn). Chill 30 minutes. Repeat for 3 turns total, chilling between each.
Shape: Roll to 1/4-inch thick, cut into triangles (4-inch base, 8-inch height). Stretch base, roll toward tip, place seam-down on parchment-lined trays.
Proof: Cover lightly, rise 1–2 hours until doubled, puffy, and jiggly.
Bake: Preheat to 200°C (400°F). Brush with beaten egg. Bake 18–22 minutes until golden. Cool 10 minutes.