PREP: Line an 8x8-inch pan with parchment paper, leaving an overhang for easy removal. Set aside for now. Remove 1 heaping cup of whipped topping and set it aside.
PUDDING: In a large bowl, combine both pudding mixes and milk and briskly whisk together for about 1 minute or until smooth. Take the remaining whipped topping and gently fold it into the pudding mixture until completely incorporated.
LAYER THE CAKE: Place cookies in the prepared pan, breaking them apart as needed to create an even layer. Top with half of the pudding mixture and use the back of a large spoon to gently spread into an even layer. Top with more cookies. Add the rest of the pudding, again spreading it into an even layer. Add the last layer of cookies. Finally, spread the reserved whipped topping in an even layer. Cover the cake and refrigerate for 24 hours. (This cake takes a little longer than most icebox cakes for the cookies to soften because they're so thick!)
SERVE: When ready to serve, cut the cake into squares (see Note 4) and, if desired, use a microplane to grate a bit of lemon zest on top. Alternatively, crumble any leftover cookies on top. Enjoy slices cold, straight out of the fridge.
STORAGE: We recommend eating the cake on days 2 or 3. It can become watery after that. To store the cake, serve it directly from the fridge. Any leftovers should be returned to the fridge promptly. We do not recommend freezing.
Notes
Note 1: Loorna Doone Shortbread Cookies: These cookies are used as the base of the cake and provide a sweet, buttery flavor. They soften when layered with the pudding mixture, turning to become a cake-like consistency. While we highly recommend using two boxes of them for this recipe, graham crackers or vanilla wafers can be used as a substitute.Note 2: Instant lemon pudding: Use two boxes of regular instantpuddingmix (not sugar-free, not low-fat) of instantpuddingmix–not the cook-and-serve type. Also, keep an eye out for the pudding box size. Get the small 3.4-ounce boxes. When using the pudding mix, follow my specific recipe directions — don’t prepare it according to package instructions. Note 3: Frozen whipped topping: Cool Whip is a common brand of frozen whipped topping that we use in our test kitchen. Be sure to use regular, not sugar-free/reduced-fat which can become watery. Or make your own; our recipe for homemade whipped cream is in the blog post!Note 4: Make clean cuts: When cutting the cake, use a sharp chef’s knife and make decisive cuts. Run the knife under warm water, dry it on a kitchen towel, and repeat for each cut.