Combine all the ingredients into a food processor or blender and blend until everything is pureed. If the paste is still chunky or too thick, you can blend the it with more water, 1 Tablespoon at a time.Once the black sesame is pureed, transfer it to a shallow pan and freeze until semi solid. Scoop filling with a Tablespoon and roll each into into 1 inch balls. Freeze until needed.
For the mochi:
In a microwavable bowl, mix together the glutinous rice flour, ube powder, and sugar until evenly combined. Add the water and mix until everything is incorporated and the mochi batter is smooth. Wrap the bowl with plastic wrap and poke a few holes on the plastic. Microwave* the mochi batter for about 2 minutes on high power, or until the mochi turns kind of translucent and brighter in color. You can also test it by poking the center of the mochi with a wet chopstick, and if it comes out clean, it's cooked.*Alternatively, if you wish, you can steam the mochi batter in a steamer for 15-20 minutes until the mochi is cooked through and looks kind of translucent.*
While the mochi is hot, fold the mochi onto itself a couple of times with a rubber spatula to work in the skin that formed on the surface. Transfer the mochi onto a sheet pan generously dusted with cornstarch and dust the mochi with more cornstarch. Let the mochi cool slightly, just until comfortable enough to handle.
While the mochi is hot and pliable, pinch off a 2 inch ball and flatten it as evenly as possible to about ¼ inch thick. With a pastry brush, brush off the excess cornstarch on both sides of the mochi. Place a pre-balled black sesame filling in the center of the mochi and pinch together the edges to seal the filling.Place the filled mochi into cupcake liners and repeat with remaining mochi.
You can garnish the mochi with sesame seeds by dampening the outside with some water.Enjoy!
Notes
Mochi is best eaten the day of, but if you have leftovers, you can store them in airtight containers at room temperature for up to 3 days.