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