Wells first of all it’s not a tweak it’s an application available from the App Store for free.
Its an application which will allow to create an icon on your home screen for any of your FaceTime contact rather a shortcut. After downloading this app from the store. Just hit the app and search your contact and after adding it gives you a chance to select a pic from your Photo Gallery or a Red, Blue icon for the contact. After that there will be a magical stick (Harry Potter thing :P) at the right, after hitting that you will be placed to the Safari and you will get a screen shot to “Add To Home Screen” and then your are done. Below is the complete image explanation
And you are done. Download 1TapFaceTime from here.