Pour the sugar into a cold skillet and place over medium-low heat. Slowly melt the sugar, swirling the pan occasionally, until the sugar turns a deeper amber and smells nutty. Take the skillet off of the heat and immediately whisk in the hot water. The mixture will bubble furiously. Keep whisking until any seized caramel dissolves completely.
Return the skillet to the stove and stir in the fish sauce, celery, lemongrass, garlic, and ginger. Bring to a gentle simmer and let cook for 5 minutes or until the celery is tender. Stir in the tofu and cook until most of the sauce is absorbed, about 3 minutes more. Take the skillet off of the heat and stir in the chiles and scallions. Set aside to cool.
Place the rice vermicelli in a large bowl and set aside. Bring a kettle of water to a boil and pour over the noodles. Let stand for 5 minutes or until the noodles are tender. Drain the noodles and rinse in cold water.
Transfer the cooled tofu mixture to a large bowl and snip the noodles into the bowl in roughly 3-inch lengths. Pour in the tahini and sesame seeds and toss to coat. Cover the filling and refrigerate until ready to use.
Take a dumpling wrapper and, with a wet finger, draw a 1-inch border around the edge of the wrapper. Place a tablespoon of the filling in the center of the wrapper and fold in half, forming a sort of taco-shape. Pinch to seal the center. Go to one side of this seal and pleat the front-facing half of the dumpling wrapper towards the center. Make two more pleats like that working your way away from the center of the dumpling. Press and seal the corner. Go the other side of the sealed center and repeat, again pushing your pleats towards the center to form a crescent shape. ** Transfer the finished dumpling to a baking sheet lined with parchment paper. Repeat until you run out of filling or dumpling skins.
From here you can simply place the baking sheet in the freezer and freeze the dumplings for 30 minutes before transferring them to a freezer bag. Or you can heat a quarter-sized amount of neutral oil in a nonstick skillet and add the dumplings. Fry the dumplings until they turn lightly golden on the bottom, then throw a 1/4 cup of water into the pan and place a tight-fitting lid over top. Steam the dumplings until tender and hot all the way through. This should take about 5 minutes.
Transfer the finished dumplings to a plate and serve with your favorite dipping sauce. I ate mine with black vinegar and sriracha.