In a medium-sized mixing bowl, whisk together the glutinous rice flour, cornstarch, the 10g of cocoa powder, and the granulated sugar until evenly combined. Pour in the milk. Whisk vigorously until you have a completely smooth, lump-free, and liquid batter. It will be quite thin, like chocolate milk.

Place a non-stick skillet or saucepan over medium-low heat. For absolute precision, it's helpful to pass the batter through a fine-mesh sieve directly into the cold pan. This catches any last tiny lumps.

Add the softened butter to the pan with the batter.

Begin to heat the mixture. As it warms, start stirring constantly with a silicone spatula or a wooden spoon. The mixture will begin to thicken. This process takes about 8-12 minutes.

Continue to stir, scraping the bottom and sides of the pan continuously. The batter will transform from a liquid to a thick paste, and then suddenly it will "seize" and come together into a single, cohesive dough ball that pulls away cleanly from the pan. Once it forms a smooth ball and no longer looks wet or sticky against the pan, it's done.

Immediately transfer the hot dough ball onto a clean work surface or a large plate. Let it cool for just 2-3 minutes, until it's cool enough to handle without burning your hands.

Now, begin to knead. Put on food-safe gloves if you have them (to prevent sticking). Stretch the dough out, fold it over, and knead it. Repeat this stretching and folding process for a good 4-5 minutes. This mechanical action is what develops the gluten-like structure in the rice flour and creates that fantastic, signature mochi chew and stretch. The dough will go from slightly lumpy to incredibly smooth, shiny, and elastic.

Once the dough is smooth, shape it into a log about 1 inch (2.5 cm) in diameter.

Spread the 2-3 tablespoons of cocoa powder for coating on a separate plate or a piece of parchment paper. Roll the mochi log in the cocoa powder until it's completely covered on all sides. This prevents sticking and adds more chocolate flavor.

Using a sharp knife or a bench scraper, cut the log into bite-sized pieces, about ¾-inch (2 cm) wide.