Feed your starter: Set aside 50 g starter and feed it with 50 g flour + 50 g water in a jar. Wait 4–6 hours until it doubles and becomes bubbly.

Make, knead, and bulk ferment the dough: Whisk together the active starter, melted butter, water, and milk. Add flour, sugar, and salt. Mix until rough dough forms, rest for 30 minutes. Knead until smooth and no longer sticky for 3–5 minutes.

Place the dough in a buttered bowl, cover tightly, and let rise at room temperature until doubled (about 4–6 hours). Refrigerate for at least 12 hours or up to 3 days.

Fold parchment to make an 8x8" packet. Slice butter ~1/4" thick and arrange on the parchment. Fold and press into a uniform packet, then chill in the fridge for 6 minutes.

Roll the dough to about 8x17" and place the beurrage on the lower half. Fold the top half over and seal the edges.

Turn dough 90° and roll to about 18" long. Fold the top third down, bring the bottom up to meet it, and fold in half. Turn 90° and roll to ~20". Final fold: fold top 2/3 down and bring bottom up over it. Roll to a rectangle of about 8x24".

Trim edges to make straight sides. Mark on the long side every 3.5" and stagger on the opposite side. Cut into triangles by connecting the marks in a zig-zag pattern.

Roll each triangle from the wider end towards the point. Place on a parchment-lined sheet with the tip tucked under.

Whisk an egg for egg wash and brush a light coat on croissants. Proof in a draft-free spot until doubled (2–4 hours).

Preheat oven to 380°F (190–195°C). Brush croissants again with egg wash and bake for 25–30 minutes until deep golden. Let rest for 15 minutes before enjoying.