No one was in the restaurant, located at 14 Waterbury Road/Route 6, or an attached business when fire broke out.
Prospect Mayor Robert Chatfield happened to be at a nearby Dunkin' Donuts around 5 a.m. when he noticed the fire at 9 and called for help.
"Smoke was coming out the front door," Chatfield described. "Seconds upon my arrival, fire blew out of the building because the windows were breaking."
Firefighters from Waterbury, Cheshire and Bethany responded and got the fire under control within about an hour.
"There was heavy fire conditions pretty much all throughout the building," Prospect Fire Chief Jason Kolodziej said. "It's pretty bad inside."
The local department and Connecticut State Police are investigating.