In a large mixing bowl, combine 500g flour and 5g sugar, stirring to mix.

Add 4g yeast and stir to distribute evenly.

Add 10g lard (or oil), then slowly pour in 280g warm water while mixing continuously. Stir until the dough comes together in a shaggy mass.

Knead the dough by hand until smooth and elastic. This should take about 8-10 minutes. The dough should be soft, slightly tacky, and smooth on the surface.

Cover the dough with a damp cloth or plastic wrap and let it rest in a warm place until it doubles in size, about 1 hour.

Cut the tofu into small, even cubes (1-1.5 cm).

Bring a pot of water to a boil, then add 1 teaspoon of salt and blanch the tofu cubes for about 1 minute to remove any bean taste. Drain the tofu and rinse it with cold water. Then, place the tofu in a large bowl and set it aside.

If using pickled mustard, finely chop 100g and add it to the tofu.

Mix the tofu and pickled mustard with 20g soy sauce, 10g oyster sauce, 20g Pixian Doubanjiang, and 5g chili powder. Stir until the tofu and seasonings are evenly combined.

Finely chop the white part of the green onions and mix it into the tofu filling.

In a small pan, heat 30g lard (or oil) over low heat until melted and hot. Pour the hot oil over the tofu mixture to release the flavors of the ingredients. Mix well.

Add the chopped green part of the onions and stir to combine.

Chill the filling in the fridge for 30 minutes. This makes it easier to handle when shaping the buns.

Once the dough has doubled in size, transfer it to a floured surface and knead lightly to release any air bubbles.

Roll the dough into a log and cut it into 20 equal portions. Roll each portion into a small ball.

Using a rolling pin, flatten each dough ball into a circle, leaving the center slightly thicker than the edges.

Place a generous spoonful of tofu filling in the center of the dough circle.

Gather the edges of the dough up over the filling, pinching the top tightly to seal the bun. Ensure the seal is tight to avoid leaks during steaming.

Repeat with the remaining dough and filling.

Place the shaped buns on parchment paper squares and arrange them in a steamer, leaving space between each bun to allow for expansion.

Cover and let the buns proof for another 20 minutes. The buns should puff slightly when they are ready for steaming.

Bring water to a boil in your steamer. Once boiling, place the buns in the steamer, making sure the water doesn't touch the buns.

Steam the buns over high heat for 15 minutes.

After steaming, turn off the heat but leave the buns in the steamer for another 3 minutes to prevent them from collapsing when exposed to cooler air.

Carefully remove the buns and allow them to cool slightly before serving.