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