Combine eggs and 1 cup sugar; beat well.
Combine walnuts, zwieback, baking powder, cinnamon, and salt in a medium mixing bowl; add egg mixture, stirring well. Add vanilla, and mix well.
Pour batter into a greased and floured 8-inch square baking pan. Bake at 325° for 40 minutes or until a wooden pick inserted in center comes out clean. Cool completely in pan.
Combine remaining 1 cup sugar and water in a small saucepan. Bring to a boil; reduce heat, and simmer, uncovered, 10 minutes. Pour hot syrup over cooled cake. Cut into squares, and serve with whipped cream, if desired.