Cut wax paper into 4 inch × 4 inch squares. Warm milk to body temperature.
Mix all dough ingredients in a bowl. Knead for about 10 minutes. Cover a bowl with plastic wrap and let rise in warm place until double in size. (about 30 to 60 minutes.)
Let's make the filling next! Mince leek and shiitake mushrooms. In a bowl, combine all the filling ingredients together. Keep in refrigerator.
Punch down dough! Divide it into 8 equal sized pieces and roll it into ball shape. Cover loosely with clean well-wrung cloth. Bring water to boil and get a steamer ready.
Roll each dough into flat circle shape with a rolling pin.
Place 1/8 of filling mixture on a dough and wrap up, then put each of them on a piece of wax paper.
When steam starts to rise from the steamer, turn down the heat. Place buns on steamer tray leaving about 2 inch between each buns.（Be careful not to get burned!! The steamer is very hot!!!) Put on the steamer lid. Turn the heat up to high and steam for 12 to 15 minutes.