Mix the flour, sugar, yeast and salt, add the eggs and milk, and work together.
Gradually add the softened butter and knead into an elastic, smooth dough (about 10 minutes by hand).
Cover and proof in a warm place for 1-1.5 hours, until doubled.
For the topping, rub together the butter, powdered sugar and flour until it forms a crumbly paste; if you want two-tone shells, mix half of it with cocoa powder.
Divide the dough into 8 equal balls, shape them into small round buns, and place them on a baking sheet lined with parchment.
Press a disc of topping onto each, then draw a shell pattern on top with the back of a knife.
Cover and proof for another 30-40 minutes.
Bake in a 180 degree oven for 18-20 minutes, until golden brown.
Source: Wikipedia – Concha (pan) (CC BY-SA 4.0), adapted and translated. This page is also licensed CC BY-SA 4.0.