Mix yeast, flour, salt, sugar, and vegetable oil. Add water until a dough forms. Knead until dough surface is smooth and elastic; about five to eight minutes. Roll over in a greased bowl, and let stand until it trebles in size (about 1 ½ to two hours).
Punch down dough, and spread out on a floured board. Sprinkle baking powder evenly on surface, and knead for five minutes. Divide dough into 12 parts. Flatten each piece into a 7.5cm (3in) circle using the palm of your hand or a rolling pin. Place two tablespoons of filling onto the centre of each circle; gather up the edges around the filling and pinch together to close the bun. Place each bun seam side down onto a square of parchment paper or aluminum foil. Cover with a towel, and let rise for about 30 minutes.
Bring water to a boil in a steamer pot, and reduce heat to medium. Transfer as many buns as will comfortably fit onto the perforated pot of the steamer, leaving 3cm-5cm between the buns. Cover steamer pot with lid (wrap towel around lid to avoid water from dripping onto buns). Steam buns over boiling water for 15 minutes. Repeat with remaining buns.