PC12 Captain reported smoke from failed EFIS resulted in expeditious descent and divert. Captain stated that mask required hindered ability to breath and communications with ATC.

Narrative

Emergency descent due to Smoke Aircraft Type: PC12Departure Airport: ZZZDestination Airport: ZZZ1 Incident Description:While cruising at FL210; smoke was detected in the cockpit of the aircraft from the instrument EADI (Electronic Attitude and Direction Indicator) SIC (Second in Command) side. The smoke dissipated gradually and did not recur for the remainder of the emergency descent. EFIS (Electronic Flight Instrument System) 2 circuit breaker popped Crew Response:Actions taken by the flight crew: 1- Oxygen mask on immediately for crew and pax (passengers)2- Asked ATC to descent below 10000 ft3- QRH 4- ZZZ2 airport was the nearest diverted to ZZZ25- After landing; Maintenance called; and the plane was grounded for inspection. Passengers and Crew:The cabin crew provided reassurance and assistance to passengers.No injuries or adverse effects were reported among passengers or crew.Things to be Improved. 1- Wearing a face mask presented challenges in terms of breathability and hindered communication with Air Traffic Control (ATC); as the transmission would often be interrupted while attempting to take a breath; resulting in an inability to hear the ATC.