"The Stanford marshmallow experiment was a study on delayed gratification in 1970 led by psychologist Walter Mischel, a professor at Stanford University.[1] In this study, a child was offered a choice between one small but immediate reward, or two small rewards if they waited for a period of time."

The joke is that in this version of the experiment, the child isn't being tested, the marshmallow is. And in this case, the marshmallow has decided to eat this one child instead of waiting until later, when it would have been allowed to eat two children.