Lightly mash half of the blueberries in a small bowl with a fork until broken down and jammy. Set aside the remaining whole blueberries.
In a large microwave-safe bowl, whisk together the glutinous rice flour, sugar, and salt until evenly combined.
Add the milk, melted butter, vanilla extract, and mashed blueberries to the dry ingredients. Whisk until you have a smooth, lump-free batter.
Gently fold in the remaining whole blueberries, distributing them evenly throughout the batter.
Cover the bowl loosely with plastic wrap and microwave on high for 2 minutes. Stir the mixture, then microwave for another 1-2 minutes until the batter transforms into a glossy, translucent, stretchy dough.
Generously dust a clean work surface with potato starch or cornstarch. Turn the hot mochi dough out onto the surface and coat the top with more starch.
Once cool enough to handle (about 5 minutes), coat your hands with starch and gently flatten the dough into a rectangle about 1/2-inch thick.
Cut into 12 equal squares using a sharp knife dusted with starch. Roll each piece gently in starch to coat all sides and prevent sticking.
Let the mochi cool completely to room temperature before serving, about 15-20 minutes. Dust off excess starch before eating if desired.
