In a microwave-safe bowl, whisk together the sweet rice flour, sugar, water, vanilla bean seeds, and salt until completely smooth with no lumps remaining.
Cover the bowl loosely with plastic wrap and microwave on high for 1 minute. Stir well, then microwave for another 1 minute. The mixture should start looking translucent.
Stir again and microwave for a final 30-60 seconds until the dough is glossy, translucent, and pulls away from the sides of the bowl easily.
While the mochi is hot, generously dust a clean work surface with cornstarch or potato starch. Turn the mochi dough out onto the surface.
Dust the top of the dough with more starch and gently pat it into a rough rectangle about 1 inch thick. Let cool for 5 minutes until warm but touchable.
Oil a sharp knife or bench scraper, then cut the mochi into 12 equal squares or rectangles. Re-oil the knife between cuts to prevent sticking.
Roll each piece gently in additional starch to coat all sides, shaking off excess. Place on a parchment-lined plate.
Let the mochi rest at room temperature for 15-20 minutes to finish setting before serving. The texture will firm up slightly as they cool.
