In a bowl, stir together the zwieback, butter, and sugar until the mixture is combined well and press the mixture into the bottom of a 9-inch springform pan. In a large bowl with an electric mixer, cream together the sugar and cream cheese until the mixture is light and fluffy. Add the flour, salt, vanilla bean, and egg yolks, beaten lightly, and combine the mixture well. Stir in the sour cream. In a bowl, beat the egg whites until they just hold stiff peaks and fold them into the cream cheese mixture gently but thoroughly. Pour the filling into the pan and run a rubber spatula through the filling in a circle about one inch from the rim to help the cake rise evenly. Bake the cake in the middle of a preheated 350f oven for 60 minutes, turn off and let the cake stand in the oven for 30 minutes. Let the cake cool completely or until it is set, in the pan on a rack. Chill the cake covered, overnight. Remove the cake from the pan and garnish it with lemon slices.
