If you consider the context, a knuckle sandwich is a punch.
Henceforth, I was not mistaken in assuming you wanted a punch to the face.
Now, considering the related context to a punch, it was only right for me to assume you wanted to fight.
*Sighing, Robin slapped his hand with two pieces of bread and squeezed some mayonnaise in his fist before slamming Laharl's knuckle sandwich into Laharl's own face*
Enjoy..