In a food processor, combine the almonds and hazelnuts. Pulse until finely chopped. Transfer to a large bowl and add the flour, spices, baking powder, and salt. Whisk until combined.
Add ½ cup of the flour mixture and the candied orange and lemon peel to the food processor. Pulse until finely chopped. Stir the chopped citrus peel into the bowl with the flour mixture. (The mixture will look like coarse breadcrumbs.)
In a large mixing bowl or the bowl of a stand mixer fitted with a paddle attachment, beat the brown sugar, honey, and egg together on medium-high speed until thickened and fluffy, about 1 minute. Add the flour mixture and mix on low speed until just combined. Scrape down the dough, cover, and let rest at room temperature for at least 1 hour or up to 8 hours.
Preheat the oven to 350F. Line two rimmed baking sheets with parchment paper.
Scoop the dough into 2-tablespoon-sized balls and place on the parchment paper about 2 inches apart. (See notes about using wafers or Back Oblaten.) Flatten them into 2½-inch wide discs using the back of a wet spoon.
Bake one sheet at a time for 20 minutes or until the edges start to brown and the centers are still soft but appear dry. Let cool for a few minutes on the pan, then transfer to a wire cooling rack.
For the Glaze:
While the cookies are still warm, make the glaze by whisking together the powdered sugar, warm water or rum, and lemon juice. Brush the glaze over the warm cookies. Decorate with blanched or slivered almonds, if desired.
Lebkuchen are best is stored in an airtight container for 2 to 3 days before serving, as the texture and flavor improve with age. The glazed cookies can be stored for up to 3 weeks.
Notes
You can use homemade candied citrus peels, store-bought, or the chopped candied citron used in fruit cakes.
Many traditional Lebkuchen are baked on large thin “communion” wafers or Back Oblaten. This helps to stabilize the cake-like cookies and offer texture. You don’t have to use them, but I like using them. Just You can order them from online retailers or on Amazon. I used the 70mm size.
Lebkuchen can also be covered in a chocolate glaze. Melt 1 cup of semisweet chocolate chips with 1 teaspoon of oil. Dip the warm cookies in the chocolate and place them on the wire rack until set.
Brandy and lemon juice add extra flavor to the glaze. You can also use orange juice or apple juice as another flavorful alternative.
Make sure to measure the flour correctly. If you add too much flour, the dough will become dry, leading to dry Lebkuchen. The ideal way to measure flour is with a kitchen scale. If you don’t have one, fluff your flour with a spoon, sprinkle it into your measuring cup, and use a knife to level it off.