Harold asks for a milkshake. SpongeBob makes one for him, however, it is frozen solid. Mr. Krabs then takes SpongeBob to the Milkshake Academy to learn how to properly make milkshakes from Captain Frostymug.
Even through all of the training, SpongeBob continues making frozen milkshakes and eventually fails. However, after saving Frostymug from a blender, he learns from him how to properly make a milkshake.
SpongeBob then uses his own body to make a milkshake, by which Harold is highly pleased.
Milkshakes also appear on the title card for "License to Milkshake."
SpongeBob makes a total of five milkshakes in "License to Milkshake." Three were frozen, one was a Krabby Patty, and one was a properly made one.
The Krusty Krab milkshakes are called "Double Dip Milkshakes," as revealed in "License to Milkshake."