Day 1: Feed your sourdough starter in the morning and let it double in size in a warm spot (4 hours at 20-22°C). If needed, feed again to ensure activity. To make Détrempé, combine flour, salt, sugar, water, milk, and active starter in a stand mixer with dough hook. Knead for 5 minutes on medium speed until a rough ball forms. Rest dough for 15 minutes. Add 50 g room temperature butter gradually, kneading for 8-10 minutes until smooth and not sticky. Shape into a ball, slash a cross on top, place in greased bowl, cover, and bulk prove 4 hours at 21-23°C until dough rises 1.5 times. Refrigerate for 3-4 hours to complete proofing and double size. For Lamination, soften butter and shape into an 18x18cm square wrapped in baking parchment; chill until firm. Roll dough on floured surface into 28x28cm square, place butter square diagonally in center, fold edges to enclose forming 20x20cm parcel; pinch edges and freeze 10 minutes. Roll parcel into 50x20cm rectangle, trim edges, fold like a letter by thirds, wrap, freeze 15 minutes, then fridge 1 hour. Repeat rolling and folding process two more times, freezing 15 minutes and chilling 1 hour after the second fold, and chilling at least 6 hours (or overnight) after the third fold. Day 2: Roll dough to 60x28cm rectangle and cut into 12 triangles. Make a small cut at base of each triangle, stretch slightly, roll from base to tip, and shape into crescents. Place on parchment-lined trays spaced apart, cover loosely with oiled cling film, and proof for 4-5 hours at 21-23°C until puffy with visible layers. Optionally create humid environment with hot water in oven to prevent skin. Chill croissants in fridge 20 minutes before baking. Preheat oven to 190°C fan. Brush croissants gently with egg yolk and milk wash. Bake for 20-25 minutes until golden and crisp. Cool at least 15 minutes before serving for best taste.