How do I remove mold from my under my bathroom sink from a leak ?

HI Ban,

Is the mold on the vanity itself or on the walls, too? Here are some ideas on this site:

https://www.wikihow.com/Remove-Bathroom-Mold

Good luck.

If it's still a very small spot, try washing with vinegar, while wearing a mask and gloves, and dry with a fan. My fear is that the water sunk into your subfloor. Leaking water can take 6 months or more to evidence itself. You may need more expert advice, before this becomes a larger problem.

The mold is on the wall. I will try washing with vinegar. Thanks.

Smother the area with a substance that will draw the wet out and dry out the mold, such as chalk powder / talc etc.