Ojisan to Marshmallow Special
おじさんとマシュマロ
An unaired episode bundled with the home release of Ojisan to Marshmallow.
Hige-san awakens to find himself at his own desk at work... in the body of a baby. To make matters worse, who should walk into his office but Wakabayashi in a school uniform!
(Source: Crunchyroll)