In a large bowl or stand mixer, whisk together the warm water, yeast and sugar. Allow the yeast to activate over about 5-10 minutes or until frothy.
In a separate bowl, whisk together the dry ingredients. Stir the vegetable oil into the dry mix until small crumbles come together.
Slowly pour the dry mix into the activated yeast and water while stirring constantly. Continue to mix until a thick dough comes together. Then use the dough hook on the stand mixer to knead the dough for about 10 minutes, until smooth and elastic. Alternatively, tip the dough out onto a flour dusted surface and hand knead for 10 mintues until smooth.
Place the dough into a bowl lightly greased with oil. Cover the bowl with a damp towel then proof until the dough has doubled in size - about 1-2 hours.*
Once doubled up in size, punch down the dough and transfer to a surface dusted with flour. Roll the dough into a long log shape then cut the dough into 12 equal sized pieces. Alternatively, if making GIANT bean buns (as seen in the movie) only cut into 4 pieces.
Gently roll each piece into a ball shape. Place the dough balls onto a baking sheet or cutting board and cover lightly with a damp towel.
Working one piece at a time, use a rolling pin to roll out each dough ball into a 4-6 inch round circle/dumpling wrapper shape. Roll the edges thinner than the center.
Dollop about 2-3 tablespoons of the red bean paste onto the center of the dough ball. Pull the edges over the filling and tuck, pleat and pinch until the filling is completely enclosed in the dough. Pinch off any excess dough (as needed) to make the dough an even thickness around the filling. Place each filled dough ball onto a square sheet of parchment. Repeat with the remaining dough and filling.**
Place the filled dough balls back onto the baking sheet or cutting board and cover again with a damp towel, move to a warm place and rise for another 15-30 minutes or until the buns are doubled in size.*
Use this second rise time to prep the bamboo steamer. Fill a wok with enough water to cover the bottom quarter inch of the bamboo steamer. Turn the heat on high and bring the water to a rolling simmer.
Once doubled in size, work in batches to steam the buns without over crowding the steamer. My steamer fit 3 buns in each basket. Close the steamer tightly and steam for 15 minutes. DO NOT OPEN THE STEAMER OR ELSE YOUR BUNS MAY WRINKLE. Carefully remove the steamed buns from the steamer and set aside. Repeat the cooking process with the remaining buns.
Allow the buns to cool for about 10 minutes to fully set the middle. Serve the steamed buns warm or room temperature for best results. To reheat, place the buns back into a preheated steamer for about 3-4 minutes. Enjoy while escaping from Yubaba's bath house.