Proof yeast: In the bowl of a stand mixer, add lukewarm soy milk. Whisk in the active dry yeast and allow to proof for about 5 minutes, or until foamy.
Mix dough: Add the sugar, salt, cooled tangzhong, bread flour, and softened butter. Mix on medium speed with the paddle attachment to bring everything together into a shaggy dough. (If working by hand, use a wooden spoon or spatula to mix the ingredients together.)
Knead with a stand mixer (recommended): Switch to the dough hook attachment and knead on medium speed for 6-8 minutes, until the dough goes from shaggy to smooth and elastic. If the dough seems overly soft or tacky, sprinkle in extra flour 2 tablespoons at a time as needed. Make sure the flour has fully incorporated before adding more.
To knead by hand: Turn the dough out onto a lightly-floured surface and knead by hand for 8-10 minutes, until dough is smooth and elastic. Sprinkle on extra flour as needed if the dough is sticking excessively to your hands or your work surface — but avoid working in too much flour. A bench scraper is also useful to scrape up any dough sticking to your work surface.