Process: In a bowl, mix flour, sugar, salt, and yeast (keep salt and yeast apart at first). Add warm milk and softened butter, mix until a dough forms. Knead 5–8 minutes until smooth. Cover and refrigerate for 1–2 hours. Place butter between parchment paper. Flatten into a rectangle, about 15 \xd7 20 cm (6 \xd7 8 in). Chill until firm but pliable. Roll out dough to about 30 \xd7 20 cm (12 \xd7 8 in). Place butter block on half the dough, fold the other half over (like a book). Seal edges. Roll gently into 60 \xd7 20 cm (24 \xd7 8 in). Fold into thirds (like a letter). Wrap and chill 30 min. Repeat rolling + folding twice more, chilling 30 min between each. Roll dough into a rectangle ~40 \xd7 25 cm (16 \xd7 10 in). Cut into 12 long triangles. Roll each triangle from base to tip, gently stretching as you roll. Place on baking sheet, tip underneath. Cover loosely and let rise at room temp (24–26 \xb0C / 75–78 \xb0F) for 1\xbd–2 hours, until puffy and jiggly.
Bake: Preheat oven to 200 \xb0C (390 \xb0F). Brush croissants with egg wash. Bake 18–22 minutes, until deep golden brown. Cool on a rack for at least 10 minutes.