In a pan, combine the milk, vegetable oil, brown sugar, and salt. Heat on low and stir constantly until the sugar has fully dissolved and the liquid begins to bubble slightly, about 3-5 minutes.

Add the high-gluten flour and stir quickly until it's fully incorporated. Continue heating on low, stirring constantly, until the flour has gelatinized and the mixture thickens. Turn off the heat and immediately add the tapioca flour and black sesame seeds. Mix thoroughly using a silicone spatula until smooth, with no dry flour remaining.

The batter should be cool by now. Beat the egg and add it to the batter in two separate batches. After adding each batch, stir with the spatula until the egg is completely absorbed before adding the next. Continue mixing until the batter reaches a "flagging" consistency (as shown in the image). When lifted, the batter should slowly fall, forming an inverted triangle, not dropping too quickly.

Transfer the batter into a piping bag. On a baking sheet, pipe out evenly sized mounds, spacing them about 2-3 cm apart to prevent them from sticking together during baking. Dip your fingers in water and gently press down any peaks on the dough. Spray water lightly over the dough before baking.

Preheat the oven to 180°C. Place the baking sheet in the oven and bake for 20 minutes. During baking, you'll notice the mochi buns expanding and turning golden brown.