Begin by heating flour, water, and milk in a small saucepan over medium heat. Continuously whisk until it forms a thick paste. It will be similar texture to buttercream frosting. Remove from heat and allow to cool.
In the mixer bowl, combine the yeast, sugar, salt and room temperature milk. Allow the mixture sit for 5 minutes.
Add the flour, butter, eggs, and tangzhong roux to the mixing bowl.
Using the dough hook attachment on a stand mixer, slowly knead the dough for about 7 minutes.
Cover bowl with a damp tea towel and place in a warm area to rise for 90 minutes.
When the dough has risen, punch out the air and remove the dough from bowl. Weigh dough ball and divide by 12. Each portion should be about 100G.
Roll each portion of dough into a ball.
Line a sheet with parchment paper and space out buns evenly. Slightly press down on buns so they are flat on top, instead of rounded balls.
Cover the sheet pan lightly with tea towel and return to the warm place to rise again. The second rise takes another 90 minutes.
Preheat the oven to 350 degrees.
Beat 1 egg with 1 tablespoon of water for an egg wash. Brush each bun with egg wash and sprinkle with sesame seeds.
Bake the buns for 25 minutes. The internal temperature should be at least 190 degrees and the tops will be a beatiful golden brown.
Remove buns from sheet pan and cool on cooling rack for 45 minutes. Once cooled, store in 2 plastic bags, half dozen in each.