01 - Whisk together instant lemon pudding mixes and cold milk in large mixing bowl for 2 minutes until thickened.
02 - Beat heavy whipping cream with powdered sugar and vanilla extract in separate bowl until stiff peaks form.
03 - Gently fold whipped cream into lemon pudding mixture until smooth and fully incorporated.
04 - Arrange single layer of graham crackers in 9x13-inch baking dish to cover bottom completely.
05 - Spread half of lemon cream mixture evenly over graham cracker layer.
06 - Top with another layer of graham crackers, then spread remaining lemon cream mixture over top.
07 - Place final layer of graham crackers over cream filling.
08 - Whisk powdered sugar, lemon juice, milk, and lemon zest until smooth and pourable consistency.
09 - Pour lemon glaze evenly over top graham cracker layer, spreading with spatula as needed.
10 - Cover and refrigerate at least 4 hours, preferably overnight, to set cake and soften crackers.
11 - Cut into portions and serve chilled.