01 - Whisk together the instant lemon pudding mixes and cold milk in a large mixing bowl for 2 minutes until mixture thickens noticeably.
02 - In a separate bowl, beat heavy whipping cream with powdered sugar and vanilla extract using electric mixer or whisk until stiff peaks form.
03 - Gently fold whipped cream into lemon pudding mixture using spatula until smooth and completely incorporated.
04 - Arrange single layer of graham crackers in 9x13-inch baking dish, breaking crackers as necessary to cover bottom completely.
05 - Spread half of lemon cream mixture evenly over bottom graham cracker layer using spatula.
06 - Place second layer of graham crackers over cream, then spread remaining lemon cream mixture on top.
07 - Arrange final layer of graham crackers over cream mixture to complete cake structure.
08 - Whisk together powdered sugar, lemon juice, milk, and lemon zest in small bowl until smooth and pourable consistency is achieved.
09 - Pour lemon glaze evenly over top graham cracker layer, spreading with spatula to cover entire surface.
10 - Cover baking dish and refrigerate for at least 4 hours, preferably overnight, to allow cake to set and graham crackers to soften completely.
11 - Remove from refrigerator, cut into squares, and serve chilled.