Lightly spray a 9×13-inch baking dish with non-stick cooking spray and set it aside.

Line the bottom of the dish with a single layer of whole graham crackers, breaking some as needed to fill in any gaps.

In a large mixing bowl, combine the lemon pudding mix with cold milk. Using an electric mixer, beat the mixture at medium speed for 2 minutes until it thickens.

Gently stir in the thawed Cool Whip until fully incorporated and the mixture is smooth and fluffy.

Spread half of the pudding mixture over the graham crackers. Add another layer of graham crackers, repeat with the remaining pudding mixture, and finish with a final layer of graham crackers.

Microwave the lemon frosting uncovered for 30–40 seconds to loosen it, then pour it evenly over the top layer of graham crackers.

Cover the dish with plastic wrap or aluminum foil and refrigerate for at least 12 hours, or overnight for the best flavor and structure.

Slice the cake into squares and serve chilled, garnished with fresh berries or a sprinkle of lemon zest if desired.

Store the cake covered tightly with plastic wrap in the refrigerator for up to four days.