It is a purple license card with bold letters at the top that read "Milkshake License." Below this, there is text that reads "Class D" and three checkboxes labeled "Chocolate," "Vanilla," and "Strawberry." At the bottom, SpongeBob's name is signed in cursive writing.
Role in series
"Yours, Mine and Mine"
SpongeBob uses his milkshake license as ID to enter Patrick's rock. Patrick deems it fake, thus denying him access.
"License to Milkshake"
SpongeBob discovers that his license had expired years ago and goes back to the Milkshake Academy to get a new one.
SpongeBob's milkshake license expired around the time of "Selling Out." This could be a possible explanation as to why his license expired.