You can't directly use an emoji as a favicon, a favicon needs to be an image.
However, you can get an image of the emoji using EMOJICDN:
<link rel="icon" href="https://emojicdn.elk.sh/🍉" />
Just replace the
🍉 emoji with any emoji you'd like.
Follow my twitter (@benborgers)
to see the work-in-progress of coding projects I’m working on!