Benjamin William Day, 20, changed his plea to guilty on the second day of his trial at Wolverhampton Crown Court.
Prosecutors said Day had carefully planned the killing of Maria Saceanu on 20 December, preparing a room in a rented property in the Tividale area of the West Midlands with plastic sheeting lining the walls and floor. He had amassed a 'shopping list' of items, including an apron, a plastic visor and cleaning chemicals, intended to help him murder her and dispose of evidence.
Prosecutor Rachel Brand KC said Day had been fascinated by the US programme Dexter, which centres on a forensic scientist in a police department who knows how to cover his own tracks as a serial killer.
Day, a University of Worcester undergraduate of Yeadon Close in Redditch, Worcestershire, was 19 at the time of the murder. He had been expected to argue that he acted in self-defence and that he set up the room to film a Dexter tribute for TikTok.
After an hour-long delay to proceedings, Day returned to the dock and appeared to smirk as he entered his guilty plea. He was remanded in custody and is due to be sentenced on 4 December.