Make the tangzhong: Whisk together 1/3 cup water, 1/2 cup soy milk, and 1/3 cup bread flour in a small pan. Make sure to break up any lumps of flour. Heat the pan over medium, stirring constantly with a heat-proof spatula, until the mixture thickens to the consistency of wallpaper paste. Remove from heat and allow to cool slightly.
Proof yeast: Add the 1 cup lukewarm soy milk to the bowl of a stand mixer. Whisk in the 4 ½ teaspoons yeast and allow to proof for ~5 minutes, or until foamy.
Mix the dough: Into the stand mixer bowl, add the cooled tangzhong, softened butter, 5 cups bread flour, 1/3 cup sugar, and 1 ½ teaspoons fine salt. Mix together on medium speed with the paddle attachment until the ingredients come together in a shaggy dough.
Knead dough: Swap out the paddle attachment for the dough hook. Knead 8-10 minutes, or until the dough is smooth and elastic. The dough should be clearing the sides of the bowl. If it is excessively wet or sticky, add in extra flour about 2 tablespoons at a time, until the correct texture is achieved. For each addition, mix until the flour fully incorporates before adding more. If kneading by hand, it may take a few extra minutes.