The member of staff at the Rising Sun branch of Harvester in Dartford Road was forced to hand over cash during the terrifying ordeal.
The robber - who struck after the restaurant had closed - is believed to have been armed, but police have not said if a weapon was involved.
Angela Lawrence, who said she lives at the Harvester, said on Facebook her boyfriend was the victim.
She said the robber "threatened to stab him if he didn't hand over the money."
On another post, she explained her boyfriend was "cashing up" when the incident took place.
"We were not open at the time and no one was physically hurt, although our manager was shaken by the robbery..." - restaurant spokesman
Ms Lawrence shared a CCTV still she claimed was the robber and asked people to share it in an effort to bring him to justice. The post has since been taken down.
Mitchells & Butlers, which runs the Harvester chain, said it was glad staff had not been injured.
A spokeswoman said: "Fortunately, we were not open at the time and no one was physically hurt, although our manager was shaken by the robbery.
"We are now helping the police with their investigation."
A police spokesman said: "We are investigating a reported robbery at the Rising Sun Harvester in Dartford.
"It is reported that between 11.30pm and midnight on Saturday, a member of staff was threatened and forced to hand over a quantity of cash to a man who wore a scarf over part of his face. Inquiries are ongoing."
Anyone with information about the raid is asked to phone police on 101.
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