After sinking to waist height and becoming caught in the slowly hardening sludge at the site in Pennsylvania on Thursday, colleagues were unable to get the pair out.
Firefighters were unsuccessful as well, and were forced to cut a hole in the tank to drain its contents, which took more than an hour.
The workers were transferred to the hospital, one by air ambulance, although there was no sign that either of them had been hurt.
Before tempering, milk chocolate is usually melted at roughly 45°C, which is somewhat higher than the recommended safe bath temperature but unlikely to cause scorching unless kept for a long time.
The workers used protective clothes that may have kept them safe from burns while the chocolate cooled around their bodies.
Mars Wrigley, which makes M&Ms and a variety of other popular sweet snacks like Snickers and Twix, runs the site.
‘We can confirm both persons have been transported off-site for further evaluation,’ a company spokeswoman told NBC on Friday.
‘We are actively monitoring the issue, and our primary emphasis is aiding emergency teams on site,’ the business previously stated.
An Illinois foundry worker died instantly earlier this week after tripping and falling into a 1400°C molten iron crucible.
After it was revealed that Steven Dierkes, 39, had not been properly qualified to use the equipment, an investigation was initiated.
A worker cleaning an industrial bread maker at a North Carolina plant died last month after becoming trapped in the machine.
Firefighters dismantled the machine and rescued Virginia López Severiano, 44, but she died in hospital from her injuries.