Preheat oven to 350 degrees.

In a large bowl, beat egg whites until stiff. In another bowl, beat yolks with sugar until thick and pale in color. Mix in vanilla extract. In another bowl, mix together nuts, zwieback, and lemon peel. Gradually fold beaten egg whites into yolk mixture and fold in nut mixture. Pour batter into a greased 13x9x2-inch pan and bake for 1 hour, or until cake tester comes out clean.

While the cake bake, make the syrup

In a saucepan, combine sugar or honey, water, cinnamon, and lemon. Bring to a boil, and simmer until sugar is dissolved, about 5 minutes. Remove from heat and stir in rum.

Immediately pour hot syrup over hot cake. Cover with aluminum foil. After cake has cooled completely, cut into diamond-shaped pieces or squares. It’s also fine to bake it in a 9-inch cake pan.