Barcelona earned a 2-0 victory at Cornella in the Copa Del Rey on Thursday evening.
Barca won their second penalty of the match 12 minutes before the end following a foul by Moreno on Clement Lenglet, however Dembele stepped up and missed.
The match ended at normal time 0-0 and promptly went into extra time.
Dembele atoned for the penalty miss two minutes from the resumption after Roman directed his powerful strike from a pass by Pedri in off the underside of the crossbar.
Cornella finished with ten players in the final minute of the match when Estelles received a second yellow card following a lunge on Araujo.
The Catalans made sure of progression when Martin Braithwaite latched onto a square ball from Pedri to tuck home.