If you are not familiar with lebkuchen, they are basically soft German gingerbread cookies. Like other gingerbread, they have an array of flavourful spices like ground cinnamon, cloves, and ginger.
In a saucepan heat brown sugar, milk, water and butter on medium heat. Stir to combine until the butter is melted and the sugar dissolves. Take off heat as the mixture just starts to come to a boil.
Add flour, gingerbread spice mix and salt and stir with a wooden spoon or spatula until combined. Return saucepan to the stove. Continue heating the dough while stirring until it thickens and starts to pull away from the sides.
Set saucepan aside in a cool place (outside if you live in a cold climate) until the dough is no longer hot so it won't cook the egg white in the next step. But don't chill so long that the dough gets too stiff to stir.
In the meantime, beat the egg white until stiff. When the dough as cooled, stir/knead in the stiff egg white, baking soda and liquor.
Prepare several sheets of parchment paper. Divide the dough into 4 portions. Place each dough portion on a piece of parchment. Place another parchment paper sheet on top and roll the dough out to a disc of about 1/4 inch thickness.
Place the rolled out Lebkuchen dough discs in the freezer or fridge until thoroughly chilled through.
Preheat your oven to 350° F and prepare 2 baking sheets.
Flour your work surface. Then take one chilled dough sheet at a time and, adding only as much flour as needed to keep it from sticking, roll to about 1/8-inch thickness.
Cut out desired shapes. Dip the cookie cutter in flour between cuts. Place the cookie cut outs on a baking sheet and decorate with nuts and seeds, if desired.
Bake one sheet at a time for about 15 minutes. Smaller shapes may bake faster, larger ones take longer. The cookies are done when the tops are opaque and the bottoms slightly browned.
In a small bowl whisk together the egg yolks and water. Pull the finished cookies from the oven, and immediately brush each cookie with a thin layer of the egg wash.
Transfer the cookies to a wire rack to cool completely. Store in an airtight container. Cookies also freeze well in Ziploc bags or airtight containers.