01 - Whisk together instant lemon pudding mixes and cold milk in a large mixing bowl for 2 minutes until thickened.
02 - Beat heavy 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 base.
06 - Top with another layer of graham crackers, then spread remaining lemon cream mixture over top.
07 - Arrange final layer of graham crackers over cream filling.
08 - Whisk powdered sugar, lemon juice, milk, and lemon zest until smooth and pourable.
09 - Pour lemon glaze evenly over top layer of graham crackers, spreading with spatula as needed.
10 - Cover and refrigerate for minimum 4 hours, preferably overnight, allowing cake to set and crackers to soften.
11 - Slice into squares and serve chilled.