The SpongeBob Got Chocolate Milk? commercial aired on February 24, 2001. The ad features SpongeBob and Patrick drinking chocolate milk to get milk mustaches. While Patrick easily succeeds, SpongeBob keeps absorbing the milk until he decides to shove his own head in the milk glass.
An image (shown here) was uploaded on the NickStory Discord server by an anonymous user. The image is the only footage of the commercial.
The commercial.: https://www.youtube.com/watch?v=0On_quxxljY
MrBones’ video on the subject.: https://www.youtube.com/watch?v=MGITN7dOBHU
ObscureMediaTV's video on the subject.: https://www.youtube.com/watch?v=k2mDFbLOcwk