The employee, who works at McVities Cake Company in Halifax, had scaled a two-metre stepladder and was trying to clear a blockage inside a mixer used to make flapjacks.
Although the machine had been switched off its blades were still rotating, and access to these moving parts caused the incident.
HSE inspector Rachel Brittain said: “An incident like this should not happen in any company, but taking place is such a large-scale food manufacturer such as this is totally unacceptable.
“Preventing access to moving parts and fitting guards is an elementary precautionary measure and inexpensive. By not putting these measures in place United Biscuits failed to fulfil its duty of care to its employees.”