To The Invisible Jet!G 20

This one was pretty tough to find, but the invisible jet is located on Level 6 - Cloud Cuckoo Land. At the start of the level, move over to the right until you see some rainbow bricks. Destroy and rebuild these bricks as UniKitty to create a launching pad. Jump on to this pad to be sent to some dark clouds. Once you're there, simply target the empty space to your right and destroy it.

Here's a quick video I made showing where this is located: