The ad, written and directed by Fallon's Juan Cabral, will break tonight on Channel Four.
But it appeared on the video-sharing site last night, almost 24 hours before its TV debut.
It is the second ad in the "Glass and a Half Full Productions" series for Cadbury's, following the hugely-successful 'gorilla' spot, which featured Phil Collins' Something in the Air Tonight.
Phil Rumbol, Cadbury's marketing director, said: "What we are focused on is the response. The key question we ask is: 'Will people see this and have a moment of unmistakeable joy?' That is the acid test."
The ad will air in the ad break of Grand Designs at 7.25pm on Channel 4, followed by showings during ER at 9.05pm and CSI: New York on Five at 9.10pm.
Did you have a moment of unmistakeable joy when you watched the ad? Comment below.