How do you approach the management of basal cell carcinoma with single lymph node involvement?

If all the tumor has been resected, I would recommend adjuvant XRT on the nodal basin, depending on the age of the patient. For very old patients or patients with comorbidities, observation might be warranted.

If there is remaining BCC visible on examination or scans, I would treat systemically with...

Very unusual. Bad behavior. Treat like an SCC.

In such an unusual situation, I would certainly advise histologic confirmation prior to treatment.