In a large bowl, mix the yeast, warm water and cane sugar. Set in a warm place for 10 minutes.Add the flour, vegan egg and grapeseed oil to the yeast mixture. Stir the mixture. Knead the dough until smooth and elastic (you can use stand mixer). Cover the dough with a damp cloth and set in a warm place for 2 hours.
Filling
In a bowl, soak mushrooms in hot water for 30 minutes. Drain, rinse and finely chop.Heat a wok over high heat. Add 1 tbsp of grapeseed oil, add mushrooms, celery (chopped), carrots (shredded) and mushed chestnuts. Fry for 3-5 minutes. Set aside. To make the sauce: in a small bowl combine all the ingredients. Pour the sauce into the wok, and cook for 2 minutes. Add fried vegetables and cook for 5 minutes.
Buns
To make baked buns: roll the dough into a log. Cut into 16 pieces. Roll each piece into a ball. Press down on the ball of the dough with your palm, use the fingers to shape into a dome. Hold the dough in one hand and place 1tbsp of the filling in the center with the other hand. Gather the edges of the dough and pull up, pleat the top until the filling is completely sealed.Place the bun, sealed side down. Place the buns in a warm place for 30 minutes. Spray buns with a mist of water and brush with vegan egg wash. Place in the oven and bake for 20-25 minutes. Remove from the oven. Brush buns with sesame oil. Serve warm.