In a medium, heatproof bowl, whisk together the glutinous rice flour, cornstarch, and 13g of sugar. Gradually whisk in the 160ml of milk until you have a completely smooth, lump-free batter.
Sift the mixture once to remove any lumps.
Cover the bowl tightly with plastic wrap. Poke a few small holes in the wrap to allow steam to escape.
Place the bowl in a steamer set over boiling water. Steam for 20 minutes. The mochi is done when it looks opaque, is completely set, and no liquid batter remains.
Carefully remove the bowl (it will be hot).
Immediately add the 10g of softened butter to the hot mochi. Use a silicone spatula to fold and stir until the butter is fully melted and incorporated. The dough will be very sticky at this stage.
Let the mochi dough cool until it's safe to handle but still warm.
Put on a pair of clean food-safe gloves (highly recommended to prevent sticking).
Lift, stretch, and fold the mochi dough for 2-3 minutes until it becomes smooth, elastic, and less sticky.
Divide the dough into 6-8 equal portions and shape each into a small, flat disc about 0.5cm thick. Set aside covered.