Add sugar and water in a small bowl and stir until sugar dissolved. Sprinkle over the yeast, whisk lightly and set aside for 5 minutes until the yeast starts to get foamy.
Transfer the dough to lightly floured work surface using the 1/4 cup flour. Incorporate the 1/4 cup flour little by little in the dough while kneading.
Knead for 15 minutes until remaining flour is incorporated and the dough is smooth and elastic.
Bring water to boil, then reduce to medium heat before placing the buns. Steam buns for 15-20 minutes.
After steaming, turn of heat then let it sit for 5 minutes before lifting the lid, otherwise the buns might shrink because of sudden change in temperature.