Preheat oven to 350 degrees F (177 degrees C) with rack in center of oven. Line two baking sheets with parchment paper. Set aside.

In the bowl of your electric mixer (or with a hand mixer), cream the butter. Add the sugar and beat until light and fluffy (about 2 minutes). Beat in egg yolk and vanilla extract.

In a separate bowl, whisk together the flour, salt, and ground cinnamon. Add the dry ingredients to the egg mixture and beat until incorporated. Add the currants and chopped mixed peel and enough milk to give a fairly soft dough.

Transfer the dough to a lightly floured surface and roll out to a thickness of 1/4 inch (5 mm). Cut into shapes using your favorite cookie cutter. Place on the prepared baking sheet.

Bake in the preheated oven for about 8 to 10 minutes. While the cookies are baking, in a small bowl, lightly beat the egg white. Remove the cookies from the oven and brush the tops of the cookies with the lightly beaten egg white. Sprinkle with a little granulated white sugar (if desired) and return to the oven for another 5 minutes, or until lightly browned. Remove from oven and place on a wire rack to cool.

Makes about 24 - 3 inch (7.5 cm) cookies.