Grandma Jo called this her ‘no-oven miracle’—one pan, a quick chill, and it’s gone before the dishes are done.

Clear a space in your fridge for a 9×13-inch glass baking dish, since this cake sets in the refrigerator instead of the oven.
In a large mixing bowl, add the instant lemon pudding mix and cold milk. Whisk for 2 to 3 minutes, until the mixture is smooth and thickened. If using, whisk in the lemon zest and lemon juice for extra brightness.
Gently fold in half of the whipped topping (about 4 oz) into the pudding mixture using a spatula, just until no streaks remain. This lightens the filling and helps it set into fluffy layers.
Lay a single layer of graham crackers on the bottom of the 9×13-inch glass baking dish, breaking pieces as needed to fully cover the bottom. Neat rows make those pretty stacked cracker edges when you slice it.
Spread about one-third of the lemon pudding mixture evenly over the graham cracker layer, smoothing it all the way to the edges so the crackers are fully covered.
Add another single layer of graham crackers over the pudding, again covering the entire surface. Repeat with another one-third of the pudding mixture, smoothing it out.
Finish with a third and final layer of graham crackers, then spread the remaining pudding mixture over the top, making sure no crackers are exposed.
Spoon the remaining whipped topping over the final pudding layer and gently spread it into a fluffy, even layer. This becomes the smooth, creamy top you’ll see through the glass dish.
Cover the dish tightly with plastic wrap or a lid and refrigerate for at least 4 hours, or overnight if you have time. This chill time is when the “no-oven miracle” happens—the graham crackers soften into a cake-like texture and the layers set.
When ready to serve, remove from the fridge and use a sharp knife to cut into neat squares, wiping the knife between cuts for clean layers. Serve directly from the glass dish, showing off those creamy lemon and graham cracker stripes.
Store any leftovers covered in the refrigerator for up to 3 days. The layers will continue to soften, which honestly makes it even better by day two.
Variations & Tips
To save even more time on a busy weeknight, you can mix the pudding and milk in the morning before work, stash it in the fridge, and then assemble the layers quickly after dinner so it’s ready by the time you’re done with the dishes. For a lighter version, use sugar-free pudding mix and light whipped topping, and swap in 1% milk; just know the layers may be a bit softer. If you love extra lemon, add more zest or another tablespoon of lemon juice, or garnish the top with thin lemon slices right before serving. For a berry twist, scatter a handful of sliced strawberries or blueberries between the pudding layers as you build the cake. You can also make mini versions in individual glass containers or meal-prep containers for easy grab-and-go desserts or lunchbox treats. If you only have a smaller pan (like 8×8), halve the recipe and expect slightly thicker layers. To dress it up for company, sprinkle crushed graham crackers or a light dusting of lemon zest over the whipped topping just before bringing it to the table.