Pour the flour into a large bowl and divided the flour into 3 portions. Pour the caster sugar, fine salt and instant yeast separately into different portions. Using a wooden spoon to stir the starter and mix well.
Pour the warm milk and beaten egg into the starter, stir evenly; add in the butter and continue stir until the starter is smoother and more gluten developed.
Transfer the dough into a lightly greased bowl. Cover and let the starter rest in a warm place to fermentation for 1 hour.
After 1 hour, the dough been double up in size. It is very soft and found some small stomata when pull it up. Sprinkle some flour on the starter, fold lightly and repeatedly from outer to inner in clockwise (not knead it). May add some flour if the starter is too sticky, until it become a dough.
Pour the dough on the table with sprinkle flour, press wide the dough lightly and divide into 16 portions.
Flatten the small dough and form it to a small ball. Set aside and cover with a wet towel and rest for 10 minutes, allow the dough to relax.
Roll the small ball of dough flat, apply ½ teaspoon of filling on the flat dough. Fold the dough in half into a half moon shape, stick the corners of the dough to as like resemble petals. Repeat the same way for the others dough.
Apply ½ tablespoon of butter to a round baking mold. Put all the 16 pieces of doughs nearby the edge. Cover with food plastic wrap and rest 20 minutes for last fermentation.
After 20 minutes, put the baking mold into the inner pot, cover up the lip, baked at 160 °C for 35 minutes to complete.