Chop the filling ingredients into smaller bits.
Prepare the dough. First, heat ¾ cup of water in a pot or microwave, then stir in the sugar until dissolved. Measure flour in a large bowl. Using a pair of chopsticks or spatula, stir the flour while adding the hot sugar water until a lumpy dough forms. If the dough is too hot to handle, cover the bowl with a lid and let it cool for 5 minutes before you continue. Dab your palm with the water, then gather the dry spots of flour in the bowl and knead into a soft dough. Add more water as you go. Once you get a smooth dough, add 2 teaspoons of oil and knead until incorporated. Cover the bowl with a lid and rest for 15 minutes.
Prepare the filling. Set a non-stick skillet over medium heat. Add 1 tablespoon of oil and sauté mushrooms, carrot, and tofu until the tofu turns golden brown. Next, add the cabbage and season with salt, Chinese 5-spice powder, mushrooms seasoning, and sesame oil. Stir-fry until the cabbage is as crisp as you like. Taste and season with more salt if needed. Turn off the heat and fold in the chopped scallions. Set aside to cool.
Assemble the bun. Divide the rested dough into 9 equal portions, about 50g each. Grease your palms with oil, roll dough into a ball, and poke a hole in the middle. Then, using your fingers, press the edges to thin it out into a cup-like shape, about 4-inches in diameter.
Then, cup your fingers on your non-dominant hand and place the wrapper on your palm. Top with fillings and press it onto the wrapper with the back of a spoon. Bring the sides to the middle and pinch to seal it tightly. Carefully press the bun with both of your palms to create a round disc. Repeat with the remaining dough. Cover the wrapped bun with a towel to prevent them from drying out.
Cook the bun. Brush the smooth side of the bun with water and sprinkle it with sesame seeds. Heat a non-stick skillet over medium-low heat and add a drizzle of oil. Place the buns into the hot skillet, sesame side down, slightly apart in one layer. The buns will expand during cooking. Cook in batches of two if you have a smaller pan.
Cover and pan-fry until golden brown, about 3-4 minutes. Uncover, flip and cover to cook this side for another 3-4 minutes. The steam will slowly cook the bun from inside out. If it appears to brown too quickly, lower the heat. Fully cooked through buns will look matte and when you press the side of the bun, it should bounce back immediately. If not, continue to pan-fry until they are done.
Serve warm with a side of your favorite chile sauce.