Scalded dough: Combine the ingredients together. Stir with a wooden spoon until a dough forms. Cover and refrigerate for 12 hours (overnight).
Dough: Combine the milk and water in a small pot, bring the mixture to a simmer and let cool till lukewarm.
Mix the bread flour, all-purpose flour, sugar, salt and instant yeast together. Pour in the lukewarm milk-water mixture and mix with a wooden spoon to form a dough. Add in the egg and mix well.
Turn the dough out onto a lightly floured tabletop. Add in the cold starter dough and knead for 10 minutes. Knead in the butter in 3 batches and knead for another 15 minutes until the dough is smooth and elastic.
Grease a bowl lightly with oil/butter. Smooth the surface of the dough and place it in the bowl. Cover with a tea towel and let the dough rise in a warm and draft-free place for 1 hour until double in size.
Punch the dough a few times to release the gas. Divide the dough into 8 portions (about 60 grams each) and roll into a ball. Cover and let rest for 15 minutes.
Filling: Flatten the dough and wrap a ball of red bean filling inside. Shape the dough whichever way you like.
Place the buns on a tray (preferably lined with parchment paper), spacing them at least 2 inches apart. Cover loosely with tea towel and set aside in a warm and draft-free place for another 1 hour to let it rise.
Finishing: Preheat the oven to 180 degrees Celsius.
Whisk the 1/2 egg and milk together. Brush the dough with egg wash and sprinkle with sesame seeds. Bake for 15 to 20 minutes until the top is lightly golden. Serve warm.