Preheat oven to 375℉.
Grease a 12" x 7" or 13" x 9" pan; set aside.
Make the syrup: in a small sauce pan, boil water and 1 c. sugar with lemon slice for about 15 mins, until syrup is slightly thickened.
Refrigerate cooked syrup until cold.
Beat egg whites until they are stiff and form mountain peaks; set aside.
In a large bowl, cream egg yolks very well, gradually adding ½ c. sugar, until egg yolks are pale yellow in color.
In another bowl, stir bread crumbs, walnuts, cinnamon, and baking powder until mixed.
Combine dry ingredients with egg yolk mixture and stir gently; then add melted butter to batter and again stir gently.
Finally, fold beaten egg whites into batter.
Pour batter into prepared pan and bake at 375˚F for 15 mins.
Turn oven down to 350˚F and continue baking for another 20 mins, or until a toothpick inserted in center comes out clean.
Working quickly, remove cake from oven, prick top with bamboo stick or fork, and pour cooled syrup over hot cake.
When cool, cut in desired shapes; diamond shapes are traditional.
A dollop of whipped cream and fresh berries is a nice accompaniment.