Nekopara: Koneko no Hi no Yakusoku

About half a year before the opening of the patisserie, La Soleil where the catgirls work with Kashou. Back when Chocola and Vanilla were still kittens when they first arrived at the Minazuki household and before they opened up to the other catgirls. It's then that they gradually began to call Kashou "master", experienced their first Christmas, and shared a promise with Kashou.

(Source: Nekopara Extra OVA website via AniDB)