To a large bowl, add instant lemon pudding mix and 1 1/2 cups cold milk. Whisk until smooth and fully combined. Add 4 ounces of the whipped topping to the pudding mixture and fold it in with a silicone spatula.
Lay a layer of graham cracker sheets along the bottom of a 9×13 pan to form a full layer. Spread the lemon pudding mixture evenly on top of the graham cracker layer.
20 sheets graham cracker sheets
Add another layer of graham crackers onto the lemon pudding. Spread the cheesecake pudding mixture evenly on top of the graham cracker layer.
Add a final layer of graham crackers, then refrigerate for 30 minutes.
Place the lemon frosting in the microwave for 15-20 seconds, then stir well. It should be soft and easy to spread, not hot. Spread the frosting evenly over the top of the entire icebox cake.
16 ounces lemon frosting
Remove the foil and lid completely from the frosting before microwaving or it may spark!
Cover the lemon eclair cake and return it to the refrigerator. Chill for at least 5 hours or overnight. Then serve and enjoy!
Notes
Let the whipped topping sit out for 5–10 minutes so it mixes more easily.Make sure to fully remove the foil from the frosting before placing it in the microwave so it doesn't spark. If you can't remove all the foil, place the frosting in a bowl before microwaving.If you prefer a little less lemon flavor, you can swap vanilla or cheesecake flavored frosting for lemon frosting.If the cake is still crunchy when you slice it, it just needs more chill time. Overnight works best, especially when it’s warm outside.