In a bowl, mix milk, egg, and sourdough starter. Add flour and mix until a sticky dough forms. Sprinkle salt on top and add butter (don't mix yet).
Cover and rest 30 minutes, then do stretch & folds until smooth and fully combined.
Cover and let rise until doubled (about 4–6 hours).
Divide into 6 pieces, shape into tight balls on a lightly oiled surface, and place on parchment.
Cover and let rise again 40 minutes until puffy. Brush with egg yolk + a little milk and add sesame seeds.
Bake at 350°F / 180°C for 20–25 minutes until golden. Brush with butter right after baking, cover, and let cool.
