Combine bread flour, water, milk, sugar, salt, instant yeast, and melted butter in the bowl of a stand mixer.

Mix on low speed for 9 minutes, then increase to medium-low for 1 minute until smooth, elastic, and able to pass a windowpane test.

Shape into a smooth ball, cover, and rest at room temperature for 30 minutes.

Flatten into a rectangle, wrap well, and refrigerate for 6–12 hours overnight.

Place 185 g cold unsalted butter between two sheets of parchment paper. Pound and roll into a uniform 18 × 18 cm (7 × 7 inches) square.

Refrigerate until cool but pliable (bends slightly without cracking).

Roll chilled dough into an 18 × 40 cm (7 × 15¾ inches) rectangle. Place butter block in the center, fold dough ends over to meet in the middle, and pinch seams to seal.

Double Fold (Book Fold): Roll dough to 20 × 70 cm (8 × 27½ inches). Brush off excess flour. Fold each short end inward to meet in the middle, then fold in half like closing a book. (Chill 20–30 minutes if dough feels warm or resists).

Single Fold (Letter Fold): Turn dough 90 degrees and roll to 20 × 60 cm (8 × 23½ inches). Fold one-third over the center, then fold remaining third over top like a letter.

Wrap dough and refrigerate for 30 minutes.

Roll chilled dough into a 20 × 20 cm (8 × 8 inches) square (~1.5 cm / ⅝ inch thick). Trim edges with a sharp knife to a clean 18 × 18 cm (7 × 7 inches) square.

Cut into 4 equal squares (9 × 9 cm / 3½ × 3½ inches). Stack two squares on top of each other to create 2 stacks.

Cover loosely and proof at 75–78°F (24–26°C) for 2.5–3 hours until puffy, light, and airy (reaching ~2 cm from top of pan).

Preheat oven to 375°F (190°C).

Bake for 30 minutes until deeply golden brown (internal temp should reach 200–205°F / 93–96°C).

Remove from pans and cool on a wire rack for 30 minutes before serving.