Place warm milk and water in the bowl of your stand mixer. Sprinkle the yeast on top and let sit for about 10 minutes until yeast begins to foam.
In a separate bowl, combine flour, sugar, salt, baking powder.
Add egg yolk to the wet ingredients. Begin kneading the flour into the wet ingredients on low speed for about 10 minutes until you have a soft round ball of dough. Transfer dough to a lightly oiled bowl and cover with plastic wrap or a clean towel. Let rise for about 1 1/2 hours or until doubled in size.
Line a baking sheet with parchment paper. On a lightly floured clean surface, cut dough into 6 equal sized pieces. Then, gently form smooth round balls by pinching dough together at the bottom making the tops round and smooth. Transfer to a baking sheet, cover with a clean towel and let rise another hour or so until dough has doubled in size.
Preheat oven to 375 degrees. In a small bowl, whisk egg white to loosen. Brush the tops of the buns with egg wash. Sprinkle tops with sesame seeds (optional). Transfer buns to the oven and bake for about 14-17 minutes until tops are golden. Remove buns from the oven and transfer to a cooling rack.