Thibaut Courtois made a fine save to deny Robert Lewandowski and just 20 seconds later Vinicius struck at the other end.
Rodrigo cut the ball forward, who fired a shot home despite Jules Counde’s best effort at a goal-line clearance. Benzema was on for sure, but Vinicius’ effort had already crossed the border.
The French forward was soon able to pounce after Luka Modric easily headed away from Sergi Roberto, finishing lethally from the edge of the box in an ocean of space.
Barcelona collapsed after that – Benzema scored his second from the penalty spot just before the hour mark, sending Marc-Andre ter Stegen the wrong way after Frank Casey fouled Vinicius.
Ter Stegen denied Marco Asensio and then Benzema completed his hat-trick with a fine finish after a fine dribble and pass by Vinicius.
“Congratulations to Real Madrid, they had a very good second half and they compete very well,” Xavi said.
“If you let Madrid off the hook, they will not forgive you. It is not by chance that they have won the Champions League — they have a great team.”
Madrid have won the Copa del Rey on 19 occasions, while their opponents Osasuna have never won it, reaching their second final after beating Athletic Bilbao on Tuesday.