Prepare an appropriate amount of warm water (around 30-35°C, just warm to the touch). Add the 3g of yeast to the warm water and stir to dissolve. This step activates the yeast, allowing it to perform better during fermentation.

Place the 300g of whole wheat flour into a large container. Add the yeast water, and if using sugar, add it at this step as well. Stir with chopsticks to combine the flour and water until there is no dry flour left and a shaggy dough forms. Then, use your hands to knead the dough into a smooth ball.

Place the kneaded dough into a container, cover it with a lid or plastic wrap, and leave it in a warm place (ideally at 30-35°C, like near a sunny window or in a warm oven) for fermentation. The fermentation should take about 1-1.5 hours, until the dough has doubled in size. To check if the dough has fermented properly, lightly dust your finger with flour and poke a hole in the center of the dough. If the hole doesn't shrink back and the dough doesn't collapse, and it shows a bubbly, honeycomb-like texture inside, the fermentation is complete.

Once the dough has fermented, take it out and place it on a kneading mat. Since the dough is full of air bubbles, it needs to be deflated. Use both hands to repeatedly knead and press the dough to expel the bubbles. Continue for 5-8 minutes until the dough becomes smooth again and has slightly reduced in size.

Roll the dough into a uniform sausage shape and cut it into 5 equal portions, each about 95g. Take one portion and knead it a few times with your hands. Shape it into a round or oval-shaped mantou. Repeat the same process with the remaining dough portions.

Place the shaped mantou into a steamer lined with parchment paper. Make sure there is enough space between each piece to prevent sticking during fermentation and steaming.

Add some warm water to the steamer to create a warm, moist environment. Cover the steamer and let the mantou ferment for another 25 minutes at around 28°C.

When the mantou has expanded to about 1.5 times its original size, lightly press the surface with your finger and let it slowly bounce back, indicating that the second fermentation is complete.

Once the second fermentation is done, place the steamer on the stove and bring the water to a boil over high heat. Once boiling, reduce the heat to medium and start timing to steam for 15 minutes.

After 15 minutes, turn off the heat but do not open the lid immediately. Let the mantou sit in the steamer for an additional 3 minutes. This step helps the mantou cook through and prevents collapse from temperature changes. After 3 minutes, open the lid and remove the steamed whole wheat mantou. Let it cool slightly before serving and enjoying.