This study will assess the safety and effectiveness of intratumoral plasmid interleukin-12 DNA injection (pIL-12) with electroporation (EP) in treatment-refractory metastatic and unresectable squamous cell carcinoma of the head and neck (HNSCC). Intratumoral pIL-12 EP is a gene therapy approach to directly induce a pro-inflammatory response within a tumor to initiate and/or enhance anti-tumor immunity.