Preheat oven to 350 F. Grease two 9-inch round cake pans with butter or cooking spray.
In a large bowl, beat cake mix with water, eggs, lemon zest, and oil. Pour batter into cake pans. Bake for 25-30 minutes or until toothpick inserted in center comes out clean. Cool cakes completely.
To make cream cheese filling: Beat together cream cheese, 1/2 cup heavy cream, powdered sugar, vanilla, and lemon juice until creamy.
In medium bowl, make crumble topping by mixing together flour, sugar, vanilla, and cold butter. Use a fork to cut mixture into crumbles.
Once cake cools, remove from pan and slice it in half horizontally. Pour enough cream cheese filling mixture to cover cut side of cake. Replace cake top. Spread a layer of cream cheese frosting over top of cake. Sprinkle top with prepared crumble. Chill at least 30 minutes before serving. Repeat for 2nd cake.