In the bowl of a standing mixer with a dough hook attached, pour in the warm milk and stir in the yeast and sugar. Let it stand until the yeast is dissolved and foamy, about five minutes.
Add the butter. Lightly beat the egg and egg white and add to the mixer as well. Add the flour and mix on low, scraping the sides as needed, for 10 minutes.
Add the raisins (or currants) and cinnamon and mix for another 5 minutes.
Transfer the dough to a greased bowl and cover lightly with plastic. Let the dough rise until doubled, about an hour.
When the dough has doubled, turn it out onto a lightly floured surface. Gently punch it down and let it rise for another 10 minutes.
Meanwhile, preheat the oven to 375 degrees Fahrenheit. Grease a 9x12 baking pan.
When the dough has risen a second time, separate it into 12 equal balls and roll them smooth. Place in prepared baking pan.
Whisk together the egg yolk and water for the egg wash. Lightly brush each dough ball with the egg wash, then bake for 20 minutes, or until golden.
Remove from oven and turn out onto a wire rack immediately. When they are cool to touch they are read to apply frosting in the shape of a cross or x on each bun.
To make the frosting, stir together all of the ingredients until smooth. Transfer to a plastic bag. Make a small cut in the corner of the plastic bag and squeeze the frosting through to make the crosses.