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