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 a separate bowl until stiff peaks form.
03 - Gently fold whipped cream into lemon pudding mixture until smooth and fully incorporated.
04 - Arrange graham crackers in a single layer in a 9x13-inch baking dish to cover the bottom completely.
05 - Spread half of the lemon cream mixture evenly over the graham cracker base.
06 - Place another layer of graham crackers over the cream, then spread remaining lemon mixture on top.
07 - Cover with a final layer of graham crackers to complete the cake structure.
08 - Whisk powdered sugar, fresh lemon juice, milk, and grated lemon zest until smooth and pourable.
09 - Pour lemon glaze evenly over top graham cracker layer, spreading with spatula if necessary.
10 - Refrigerate covered for at least 4 hours or overnight to allow cake to set and crackers to soften.
11 - Cut into squares and serve chilled for best texture and flavor.