Toast black sesame seeds in a dry frying pan over low heat until they plump up and become very easy to break when crushed with your fingers (see note 1).
Once completely cool, put the toasted black sesame seeds in a food processor. Grind them with sugar until they turn into a soft, shiny paste.
Add butter (or coconut oil/lard). Mix to combine then keep refrigerated until the mixture is firm enough to handle.
Divide the paste into 20 portions. Shape each piece into a ball. Put them back in the fridge while preparing the dough (see note 2).
Make the dough
In a mixing bowl, pour boiling water into glutinous rice flour while stirring with a spatula. Then add room temperature water (or beetroot juice for a pink dough) little by little.
Knead with your hand until a smooth, soft dough forms. Adjust its consistency by adding more water or flour (see note 3). If you are not using the dough straightaway, wrap it with cling film to avoid drying out.
Divide the dough into 20 equal pieces then roll each into a ball.
Assemble Tang Yuan (please refer to the video below)
Flatten a piece of dough into a round wrapper with your fingers.
Place a ball of filling in the middle. Gently push the wrapper upwards to wrap the filling tightly. Seal at the top completely (see note 4).
Cook Tang Yuan
Bring a large pot of water to a full boil. Gently slide in some Tang Yuan. If necessary, cook in batches as the balls expand while cooking so do not crowd the pot.
Push the balls around with the back of a spoon to prevent them from sticking to the bottom of the pot. Cook over medium-high heat. Once all the balls start to float on the surface, boil for a further minute.
Dish out (along with some cooking liquid) and serve warm. You may also make some ginger syrup separately as the serving liquid (see note 4).
Store Tang Yuan
Freeze Tang Yuan right after they are assembled. Lay them in a single layer on a tray lined with parchment paper to freeze. Once totally frozen, put them in an air-tight bag and store in the freezer.
To cook frozen Tang Yuan, follow the same cooking procedure (do not defrost).
Video
Notes
1. Do not overly toast the seeds as it would lead to an unpleasant burnt flavor.2. To make assembling easier, the filling should be quite firm. So you may put the filling balls in the freezer for a short while if they appear too soft.3. The amount of water (or beetroot juice) required varies depending on the brand of the flour. Adjust if necessary. The finished dough needs to be smooth and very soft but not at all sticky.4. The dough made of glutinous rice flour isn’t as elastic as regular dough. It may crack during the assembling (especially if it’s not soft enough). If that happens, wet the broken part with a little water then rub gently to reseal.5. Make ginger syrup:In a separate saucepan, simmer a small piece of smashed ginger and sugar in water for 3 minutes or so. Taste then adjust the flavor. Pour into the bowls with cooked Tang Yuan.