In a medium mixing bowl, combine 100g glutinous rice flour, 30g cornstarch, and 35g white sugar. Pour in 170g milk. Whisk until the mixture is smooth and the dry ingredients are fully incorporated. The batter should be thin and pourable, similar to heavy cream.
Strain the batter through a fine-mesh sieve once.
Heat a non-stick pan over low heat. Pour the strained batter into the pan. Add 20g butter.
Cook over low heat, stirring constantly with a silicone spatula or a wooden spoon. The mixture will look like a lumpy paste at first — keep stirring. As it heats, it will start to come together into a single mass. Continue stirring and folding until the dough forms a complete, cohesive ball that pulls away cleanly from the sides and bottom of the pan. This usually takes about 5-8 minutes.
Transfer the cooked dough to a clean work surface. Let it cool just enough that you can handle it without burning yourself (but it should still be warm).
Now comes the most important step for achieving that signature soft, stretchy, chewy mochi texture: knead and stretch the dough repeatedly. Fold it over itself, press it down, stretch it out, fold again. Repeat this process for 5-10 minutes. You'll notice the dough becoming smoother, glossier, and more elastic with each stretch.
When the dough is smooth, glossy, and stretchy (you should be able to pull it into a thin sheet without it tearing), it's ready. Set aside and cover with plastic wrap to prevent it from drying out while you prepare your work station.