Toast the black sesame seeds in a dry skillet over medium heat for 3-4 minutes, stirring constantly, until fragrant. Let cool, then grind to a coarse powder using a spice grinder or mortar and pestle.
In a microwave-safe bowl, whisk together the sweet rice flour, ground black sesame, sugar, and salt until evenly combined.
Add the water and black sesame paste to the dry ingredients. Whisk until completely smooth with no lumps remaining. The mixture should look like thick pancake batter.
Cover the bowl loosely with plastic wrap, leaving a small vent. Microwave on high for 2 minutes, then stir vigorously with a wet spatula.
Microwave for another 1 minute, then stir again. The mochi should look translucent and pull away from the sides of the bowl when done.
Dust a clean work surface generously with cornstarch. Turn the hot mochi out onto the surface and dust the top with more cornstarch.
Let cool for 10 minutes until just warm to the touch. With lightly wet hands, divide the dough into 12 equal portions and roll each into a smooth ball.
Place the mochi balls on a cornstarch-dusted plate and let them sit for 15 minutes to set the exterior before serving.
