In a saucepot, whisk together the ingredients for the tangzhong as it heats. Once it thickens, take it out and let cool in the fridge.
Add the flour, vital wheat gluten, sugar, salt, and yeast to a bowl of a stand mixer. whisk to combine.
Heat the soymilk until warm. Add to the dry mixture. Mix the water and cornstarch to endure there is no lumps and add it, as well. Lastly, add the cooled tandzhong.
Mix on high for 8-10 minutes until the dough starts to stick to the dough hook. Add a little bit of the butter and mix on medium speed until it incorporates into the dough. Repeat until all the butter is gone.
Cover the bowl and let rest in the fridge overnight or for atleast 8 hours.
Punch the dough down to remove air bubbles. Place on a lightly floured surface and roll about 1 cm thick. Flour the top as well. Using a 7 cm circle cutter, cut the donut circles. You should be able to get 6 donuts total. Using a 3.5 cm thick circle cutter or a bottle cap, cut the donut hole out.
Place donuts on a lightly oiled square piece of parchment paper. Be sure to leave room for the donut to grow in size.
Let proof for 30-45 minutes in a warm location.
To make the glaze, melt the butter and mix with your sifted powdered sugar. Add th vanilla extract and milk to make it thick but flowing. Let sit.
In a tall pot, heat the oil to 350°F
Using a slotted spoon, carefully place the donut and parchment paper on top and slide into the oil. Remove the parchment paper.
Fry one side of the donut for 1-2 minutes. Flip and fry for 1 minute more. Place it into the glaze to coat. Let the excess drip off and let cool on a metal wire rack. Repeat this process with the other donuts.
Let cool at room temperature and is best enjoyed while still warm.