Air carrier flight crew reported smelling an electrical burn smell followed by a LAV ECAM message. The flight crew ordered the evacuation of the aircraft at the gate.

Narrative

At ZZZ gate XXX just prior to finishing boarding. We started to smell something like an electrical burn smell in the flight deck. We started to look around and soon after we got a warning and a Lav smoke message on ECAM. We checked the first class Lav and saw some smoke and smoke starting to appear on the flight deck. I then told the flight attendants to deplane the aircraft immediately. They started the procedure of emergency deplaning via the boarding door to the jetway and terminal promptly. Then I called our operations and told them about the smoke and call the fire department. Then I and the F/O started to shut down the aircraft and secure it. After accomplishing that; we exited to the jetway after everyone was off the aircraft. We made our selves available to authorities; firefighters and mechanics for questions.Cause: What the cause of the smoke ended of being was from one the aircraft batteries in the avionics bay. The firefighters with guidance from maintenance personnel removed it.