Make the dough: Mix flour, sugar, salt, and yeast. Add warm milk and knead 10 minutes until smooth. Cover and rest 1 hour until doubled. Punch down, wrap, and refrigerate 30 minutes.
Prepare the butter block: Place butter between parchment and pound/roll into a 6x8 inch rectangle. Chill until firm but pliable.
Laminate: Roll dough to 12x8 inches. Place butter in center and fold like a letter. Roll out, fold into thirds (one turn). Chill 30 minutes. Repeat for 3 turns total.
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 and rise 1 to 2 hours until doubled, puffy, and jiggly.
Bake: Preheat to 200C (400F). Brush with beaten egg. Bake 18 to 22 minutes until golden. Cool 10 minutes.