In a microwave-safe bowl, combine glutinous rice flour, sugar, cornstarch, and salt. Gradually add milk and water, whisking until smooth.
Cover the bowl with plastic wrap (vented) and microwave for 1 minute. Stir well, then microwave two more 1-minute intervals, stirring between each.
When the mixture becomes translucent and stretchy, let it cool for 2–3 minutes.
Dust a work surface with potato starch. Transfer the dough and dust the top with starch to prevent sticking.
Divide the dough into 8–10 pieces and roll into smooth balls with your hands.
Allow the mochi balls to cool for 15 minutes. Brush off excess starch.
Drizzle with condensed milk or dust with powdered sugar before serving.