In a bowl, combine bread flour, sugar, milk powder, and instant yeast. Make a well in the center. Whisk and combine well all egg yolk and water, then add into the well of dry ingredients. Mix and knead it until half elastic, add butter and salt. Continue to knead until the dough is no longer sticky but elastic enough. You want the dough to be elastic. So if you were to take a part of it and stretch it out, you can stretch it to a very thin membrane without it breaking. When you poke a hole in the thin membrane, it should form a close to perfect circle.

Divide the dough into small pieces of dough about 40 g, make ball shape. Cover with cling wrap and let rest for 15 minutes. Deflate the dough, fill with the filling, make ball shape again and place the balls into baking pan that has been spread by butter or lined with parchment paper. Leave it for second round of proofing about 30 minutes until double in size.

Whisk the topping ingredient: egg, salt and water in a small bowl. Brush egg wash on the surface of the bun and spread chocolate jimmies or grated cheese. Bake in a preheated oven 180°F Celsius for 15 minutes or until the surface golden brown. Transfer into wire rack and let cool completely.