Tangzhong:
2 tablespoons (20g) bread flour
2 tablespoons (27g) water
4 tablespoons (60g) whole milk

Dough:
1/2 cup(120g) whole milk @ 95f (30c)
1 tablespoons (9g) instant yeast
2.5 cups (320g) Bread flour
1 teaspoon (7g) fine sea salt
2.5 tablespoons (35g) granulated sugar
1 whole egg
1 egg yolk
3 tablespoons (42g) unsalted butter, softened

Egg wash:
1 whole egg
splash of whole milk

Prep:

Make the Tangzhong. Combine milk, water, and flour in medium saucepan. Whisk over medium heat until mashed potato texture. Take ½ C Whole Milk to 95 F. Add yeast, stir, and let sit 10 minutes. 

In mixing bowl, add flour, sugar, and salt. Whisk and then fit mixer with dough hook. On low speed, slowly add milk mixture and the Tangzhong paste. Follow with the egg and egg yolk. Mix on medium low until thoroughly incorporated. 

Gradually add the butter- GRADUALLY! Let the mixer knead the dough for 5-8 minutes until smooth. 
Place seam-side down dough ball in greased bowl, cover with damp towel and place in warm area for 1 ½ hours. 

Once dough is doubled, punch down & divide into 6 even pieces (weigh to be accurate). 

To shape:  Gently stretch the piece of dough toward the center, turn over, and gently pull toward you, pressing the seams further. Make a perfect, tight ball for the perfect bun. 

Place dough balls on baking tray lined with greased parchment. Place inverted baking sheet over the top to prevent drying. Lest rise for 1-2 hours. 

Brush the entire top of the buns with egg wash. 
Bake at 190C for 16-18 minutes until golden brown. Brush with melted butter & let cool.

https://www.youtube.com/watch?v=gTGSUYMu6Ns&t=525s