Lightly toast the black sesame seeds over medium heat until you can smell their aroma. Note that the sesame seeds may start to pop, so cover the pan with a lid. Be careful not to burn the seeds; transfer them out to cool as soon as they become aromatic.
Use a mini food processor to grind the black sesame seeds until fine. Transfer the ground sesame into a wok, add sugar and butter, and stir well to form a thick paste. If the mixture is too dry, add more butter. Dish out and let cool in the fridge to make the filling easier to handle.
In a large bowl, mix the glutinous rice flour with water until it forms a smooth paste that no longer sticks to your hands. Divide the mixture into 16-20 equal balls (the size can vary depending on your preference; larger balls are easier to fill). Flatten each ball in your palm, then use a pair of chopsticks to pick up some black sesame paste and place it in the center of the flattened ball. Fold the edges to seal the dumpling, then gently roll it into a ball shape using both palms. Set aside.
Prepare the ginger syrup by boiling the water. Add the ginger and screwpine/pandan leaves (if using) and boil for 10-15 minutes over medium heat. Then, add sugar and sweet osmanthus, and boil for another 5 minutes. Lower the heat to a simmer and reduce the mixture to about 4 cups of liquid. Adjust sweetness by adding more sugar to taste, if desired.
Heat another pot of boiling water. Drop the dumplings into the hot water, and once they float to the top, transfer them to the ginger syrup. Turn off the heat and serve the black sesame dumplings in a bowl immediately.