In a large bowl, combine the warm water, yeast, and sugar. Stir gently and let it sit for 5-10 minutes until foamy.
1 cup warm water, 1 tablespoon active dry yeast, 1 tablespoon granulated sugar
Add 2 ½ cups of the flour, vegetable oil, salt, and baking powder (if using) to the yeast mixture. Stir with a wooden spoon or spatula until a shaggy dough forms.
2 ½ cups all-purpose flour, 1 tablespoon vegetable oil, ½ teaspoon baking powder
Turn the dough out onto a lightly floured surface. Knead for 8-10 minutes, gradually adding more flour if needed, until the dough is smooth, elastic, and no longer sticky.
Lightly oil a clean bowl. Place the dough in the bowl, turning to coat. Cover with plastic wrap or a damp cloth and let rise in a warm place for 1 to 1.5 hours, or until doubled in size.
Gently punch down the risen dough. On a lightly floured surface, divide the dough into portions and roll into 8 equal portions and shape into round balls for simple mantou. For smaller buns, divide into 12 portions.
To make flowers, use a sharp knife and make four cuts into the ball (positions 12, 3, 6, and 9 like on the face of a clock), not quite to the center. Gently press down on the ball and shape the four petals as you like. Set a few sesame seeds in the center.To make spirals, roll a ball of dough into a long rope, then coil into a spiral. Place the shaped buns on individual squares of parchment paper (this prevents sticking to the steamer). Arrange them in a steamer basket, ensuring there's space between each bun for expansion. Cover the steamer and let the buns rise for another 20-30 minutes, or until puffy.
Conventional Steaming: Bring water in your steamer pot to a rolling boil. Place the steamer basket with the buns over the boiling water. Steam for 10-15 minutes, depending on the size of your buns. Avoid lifting the lid during steaming.INSTANT POT Instructions: Add 1 cup water to the Instant Pot. Add the steamer basket of mantou. Cover and seal. Set to LOW pressure for 3 minutes. Turn off the heat and let the buns sit in the covered steamer for 5 minutes before carefully removing the lid. This prevents them from deflating. Transfer to a wire rack to cool slightly before serving.