A double onehanded slap
to the face with an open hand. You administer it in on motion, first forehand and then a backhand. It has to be two slaps, hence the sandwich. If you start with
the backhand and follow up with forehand it's called a "reverse knuckle sandwich" or an "upside down knuckle sandwich". The standard knuckle sandwich is served with the right hand, a
left handed knuckle sandwich is sometimes called a "lynx knuckle sandwich".
When hitting the face it is always a good idea to use an open fist. The risk for injury and jail time is less, and it is quite effective. Save the closed fist for the torso.