In a bowl of a stand mixer measure the flour (360g), dry milk powder (40g), yeast (10g), sugar (25g), salt , and half of the butter (55g).
Using the paddle attachment, with the mixer on low speed, pour in the milk and mix until you have a shaggy mass. It should look sticky, but cohesive (small spots of butter are ok), and this should take about 30 seconds.
Replace the paddle attachment with the dough hook, add the rest of the butter (55g), then knead on medium speed for about five minutes.The dough is ready when it is smooth, elastic, and sightly sticky. It can take more or less than five minutes, so please pay attention to the dough and not the timer. Transfer the dough to a large clean bowl (or leave it in your mixer bowl), cover with plastic wrap and set aside to proof for 2-3 hours.
Line a large baking sheet with parchment paper and beat the yolk in a small bowl.
Deflate the dough and divide it to 7, each approx. 105g.
Working on a clean and smooth work surface, cup a piece of dough with your hand, then move your hand clock wise for about 15-20 seconds. Do it as fast as you can.
Gently lift the dough and place it over the prepared pan. Repeat with the rest of the dough pieces, leaving about 2-inches gap around each bun.
Gently press the top of each bun to make sure the buns bake without a pointy top.
Brush with the egg wash, and sprinkle with your choice of topping.
Set aside to proof for 60-90 minutes.
About 20 minutes before baking, preheat your oven to 350°F (180°C).
Bake over the middle oven rack for 20 minutes turning the pan at the 15 minutes mark.The buns are ready when they are puffed, golden brown (not very deep), and the inner temperature is 180°F (82°C). Remove from the oven, set aside to cool for about 15-20 minutes, and serve.