Combine cake mix, water, oil, egg whites, lemon juice and lemon zest. Follow directions on box. Pour into greased and floured spring-form pan. Bake at 350 for 40-45 minutes. Cool.
For filling, mix softened cream cheese, powdered sugar and lemon juice until smooth. Whip cream until stiff peaks form. Combine by hand with cream cheese mixture.
For crumb topping, combine flour and powdered sugar, cut in butter, vanilla, and lemon juice (pastry blender or food processor).
Slice cooled cake in half. Drizzle lemon juice on each cut side. Spread half of the cream mixture on bottom half of cake, put cake top on and spread the remaining cream on top and sides. Sprinkle crumb topping on top and pat on the sides.