Cut the butter into thin slices, spread on a plate, leave it on the counter to warm up.
Crumble the yeast into a small cup, pour in warm milk, mix until completely dissolved. The milk should be pleasantly warm to the touch. If you have an instant-read thermometer you can measure its temperature, it should be about 36°C / 100°F. Milk can’t be too cold or too hot (the dough won‘t rise). Cover the cup with a kitchen cloth and let it rest for 10-15 minutes.
Add all the ingredients for the dough into a large bowl (yeast dissolved in milk, soft butter, flour, salt, sugar, egg and egg yolk). Mix the ingredients with a spoon until roughly combined. Next, knead the dough either by hand or using a stand mixer fitted with a hook dough attachment. The dough must be really well-kneaded, it will take about 7-10 minutes to knead with your stand mixer.