Hanuman as vahana or vehicle of Goddess Kali is a concept popular in Bengal. It is found in the Bengali Version of Ramayana known as Krittivasi Ramayana. The story connecting Goddess Kali to Hanuman is the famous Mahiravana episode in Ramayana.
Mahiravana kidnaps Lord Ram and Lakshman. He plans to behead the brothers as an offering to Goddess Kali.
Hanuman reaches the abode of Mahiravana and rescues Ram and Lakshman. In the process, Hanuman chops of the head of Mahiravana and offers it as sacrifice to Goddess Kali.
Goddess Kali was impressed with the devotion and dedication of Hanuman to Lord Ram.
She told Hanuman to serve her as her guardian when Ram Avatar comes to an end. As Hanuman is a Chiranjeevi (immortal), he continues to live on earth guarding Goddess Kali temples.
Thus in Kali Yuga, Hanuman is the Vahana of Goddess Kali and protector of her devotees on earth.