Dissolve the yeast in the warm milk, then add the sugar, salt, butter, and two eggs plus one egg yolk. Reserve the remaining egg white. Add the flour and knead in a stand mixer for 10 min. Remove the dough and let rise in a greased bowl until doubled.
On parchment paper, roll out the dough into a rectangle and mix the filling ingredients. Spread the filling on the dough and fold the outer thirds over the middle third lengthwise. Cut the dough in half lengthwise, twist or braid the strands together, and connect them at the ends to form a crown. Let it rise once more for 30 min.
Mix the reserved egg white with 1 tbsp water and brush over the bread. Bake at 350ºF until golden, about 50 min. While waiting, mix the confectioners’ sugar and milk together to make frosting. When the cake has cooled, decorate with the prepared frosting and top with sprinkles. Insert the trinket into the cake, slice, and serve!