In a large skillet over medium heat, add the black sesame seeds (do not use any oil). Heat for 3 – 5 minutes until fragrant, stirring often. If you bought roasted sesame seeds, just toast them quickly for 1 minute. Transfer to a large plate to cool completely, at least 1½ hours.
In a food processor or blender, add the cooled black sesame seeds. Process until fine (as fine as you can). Transfer to a medium bowl. Add sugar and mix well. Add melted butter and combine. Divide the paste into 23 portions. Roll and squish them into little balls. Transfer to freezer and chill for at least 1 hour.
When the filling is ready, work on the dough. In a large bowl, add glutinous rice flour. Carefully add the boiling water. Stirring with a wooden spoon until some clumps formed. Add cold water and stir to incorporate everything. Mix with hand. If there is still dry flour on the bottom of the bowl, it needs more water. Add 1 teaspoon cold water and mix again. If it’s still too dry, repeat the process. The dough should be smooth. Knead for a couple minutes.
Divide the dough into 23 portions, about 20 grams each. Cover the dough with a moist towel. Roll each portion into a ball and pat into a small pancake (about 4-inch in diameter). Place one frozen filling in the middle. Carefully wrap the filling with the dough and roll into a ball. Transfer to a large plate and cover with plastic wrap. Repeat with the rest of the dough.
The dumplings can be used right away. You can also keep them in the fridge in a ziploc bag if using within a few hours. If not, freeze in a ziploc bag and use within a couple weeks.
For the sweet soup, in a medium pot, bring the water to boil in high heat.
Meanwhile, put the ginger slices on the chopping board. Smash the ginger with the side of a knife to release the ginger juice and flavor.
Once the water is boiling, put in ginger slices and half of sugar (break the sugar with the back of a knife). When the sugar has melted, taste to try. Adjust the sweetness to your likeness.
Take the dumplings out of the fridge or freezer, about 2 – 3 dumplings per person. Carefully place them in the boiling liquid. Reduce the heat to low. To make sure the dumplings do not stick to the bottom of the pot, use a spoon to scoop them lightly. Wait until they float on the water. Once they are floating, cook for 2 more minutes (from fridge) or 3 – 4 minutes (from freezer). Transfer the dumplings and soup to serving bowls. Serve immediately.