1) Combine all dry ingredients: flour, salt, sugar and yeast in a bowl. Make a well in the center.
2) Whisk and combine all wet ingredients, then add into the well of the dry ingredients.
3) When all ingredients get together, knead in the softened butter. Keep kneading until the dough is smooth, not sticky and elastic.
4) Shape dough into a ball. Place in a greased bowl and cover with a wet towel or cling wrap. Let the dough sit for about 40 minutes.
5) Preheat oven to 356 degrees.
6) Transfer the dough to a clean floured surface. Deflate and divide into 12 equal portions. Cover with cling wrap, let rest for 15 minutes. Knead each little dough portion into ball shapes and let them sit for about 35 minutes to proof and double.
7) In the meantime, make the topping. Cream softened butter until smooth. Add sugar and continue to beat until fluffy.
8) Add the egg yolks, milk, honey and baking powder and combine well.
9) Sift in the flour and mix well.
10) Use some plastic wrap to roll the mixture into a log and chill it for 30 minutes.
11) Once the mixture has hardened, cut into 12 equal portions and roll in balls. Cover by a film wrap and press each portion into a flat disc.
12) Place the topping on each of the dough balls.
13) Mix together the ingredients to make the egg wash and brush the top of the dough balls with it.
14) Bake in the preheated oven for 20 minutes.
15) Remove the buns from the oven and let them rest for a few minutes.
16) The buns can either be served right away, while still warm, or chilled in the fridge and then served.