01 - Whisk together the instant lemon pudding mixes and cold milk in a large mixing bowl for 2 minutes until mixture begins to thicken.
02 - In a separate bowl, beat the heavy whipping cream with powdered sugar and vanilla extract using an electric mixer or whisk until stiff peaks form.
03 - Gently fold the whipped cream into the prepared lemon pudding mixture until smooth and fully incorporated, taking care not to deflate the cream.
04 - Arrange whole graham crackers in a single layer in a 9x13-inch baking dish, breaking pieces as needed to completely cover the bottom.
05 - Spread half of the lemon cream mixture evenly over the first layer of graham crackers using an offset spatula or the back of a spoon.
06 - Place another layer of graham crackers over the cream, then spread the remaining lemon cream mixture evenly on top.
07 - Complete the cake with a final top layer of graham crackers, ensuring even coverage.
08 - Whisk together powdered sugar, fresh lemon juice, milk, and grated lemon zest in a small bowl until smooth and pourable.
09 - Pour the lemon glaze evenly over the top layer of graham crackers, spreading gently with a spatula to cover completely.
10 - Cover the baking dish with plastic wrap or aluminum foil and refrigerate for at least 4 hours, preferably overnight, to allow the cake to set and the graham crackers to soften to a cake-like texture.
11 - Slice into squares while cold and serve chilled. Use a sharp knife dipped in warm water for clean cuts.