In large bowl, using an electric hand mixer, blend together pudding and milk until thickened. Add cool whip and blend to well incorporated.
In a 9x13 baking dish, place a single layer of graham crackers covering the bottom. (Break a few to fill in empty spaces.)
Spread half of the pudding mixture evenly over graham crackers.
Layer more graham crackers in a single layer over mixture.
Spread rest of pudding mixture over graham crackers evenly.
place 2 layers of graham crackers over pudding mixture.
Mix together all frosting ingredients using an electric hand mixer. Spread evenly over graham crackers.
Cover baking dish and place in refrigerator for 4 hours to overnight. (Decorate with thin slices of lemon, if desired.)
