01 - Dissolve yeast in lukewarm milk and let stand for 5 minutes until foamy.
02 - Combine flour, sugar, salt, and softened butter with yeast mixture. Mix until rough dough forms, then knead for 5–7 minutes until smooth and elastic.
03 - Shape dough into rectangle, wrap tightly in plastic, and refrigerate for 1 hour.
04 - Place cold butter between parchment sheets. Pound and roll into 6 x 8-inch rectangle. Refrigerate if softening occurs.
05 - Roll dough on floured surface into 10 x 14-inch rectangle. Position butter slab on lower half, fold upper half over, and seal edges.
06 - Gently roll dough into 10 x 20-inch rectangle. Fold into thirds like a letter. Wrap and refrigerate for 30 minutes.
07 - Repeat rolling and folding process twice more, rotating dough 90 degrees between each turn. Refrigerate 30 minutes after each fold.
08 - Roll final dough into 10 x 20-inch rectangle. Cut into 8 equal rectangles. Place chocolate at one end of each piece and roll tightly into log, seam-side down.
09 - Arrange on parchment-lined baking sheet. Cover loosely and proof at warm room temperature for 2 hours until puffy and doubled.
10 - Preheat oven to 400°F.
11 - Whisk egg with milk until combined. Brush evenly over proofed croissants.
12 - Bake for 18–20 minutes until deep golden brown and crisp. Cool slightly on baking sheet before serving.