Air carrier Captain reported an intermittent fume event initially noted during boarding which dissipated after engine start. The strong odor resumed after takeoff which prompted the Captain to execute an immediate and safe air return for maintenance.

Narrative

While operating [FLIGHT NUMBER] [ROUTE]; crew noted a slight and unusual odor during the boarding process. Through air vents servicing the the cockpit; a smell resembling 'slightly burned popcorn' was noted. Consult with the flight attendants revealed no initial odor in the rear cabin; though a slight smell became apparent approximately 10 minutes from push back. A visual walk through of the cabin confirmed no active source; and nothing was noted in the forward cargo bay when inspected by the Captain. Crew conducted a Maintenance consult which resulted in improvement when the Packs were shut off. Maintenance suggested that the crew monitor the situation post engine start and note any changes after shutting down the APU. When asked by flight crew; Maintenance did not see a need to conduct an initial write up in the logbook if conditions improved and no further odor was present. Post engine start and after the APU had been shut down; the odor dissipated to nil during the taxi out. Prior to reaching the runway; zero odor was present in the aircraft as confirmed by all flight crew members. With this in mind; the unusual odor appeared to be gone with no signs of any aircraft malfunction. After applying takeoff thrust and reaching rotation speed; a very strong presence of the initial odor became apparent just after wheels up. The crew immediately and unanimously opted to return to the field without delay. No visible smoke or fumes were noted along with any EICAS (Engine Indicating and Crew Alerting System) indications of malfunction. Soon thereafter; the odor again began to dissipate; however crew elected to return to the airport despite there being no more smell. With no visible indications of smoke; fire; fumes; or activation of a detection system; the crew did not declare an emergency at that time. ATC was however advised of the situation in addition to company; crew; and passengers. [FLIGHT NUMBER] returned to the field without incident. Upon landing; the smell did return once the APU was started prior to pulling into the gate. A fire department lieutenant was on scene and confirmed the presence of odor in the cockpit; resembling a slightly burnt smell. No other findings were noted and the aircraft was grounded and referred to Maintenance. No additions outside of event narrative.Crew should have performed an initial Maintenance write up despite consult with Maintenance and disappearance of odor. Additionally; consultation with the FODO (Flight Operations Duty Officer) would have been a prudent decision. I would also elect to declare an emergency in this situation.