Put the sandwich in a freezer at -25 C° and leave it there for a month.
When it gets hard as a brick, take it out and start hammering very hard on the head of the person you want to kill.
Repeat as many times as needed.
Don’t know about the reason of the murder. But if it was revenge, well, you have served it cold.
Important advice:
Before doing this to any other people, try first on your own head. Keep hammering on your head until you lose your senses. Then, when you wake up, do it again. Keep on doing that until you are 100% sure that it works.