The 17-year-old holding midfielder who joined the Blues Youth Academy five seasons ago made his FA Cup debut.
Bobby Devyne opened the scoring just before the break but another Ghanaian prodigy Bernard Mensah levelled the scoring for Watford mid-way through the second half.
However, the Blues finally sealed their place in the semi-final for the third time in four seasons.
Pappoe is one of the numerous young Ghanaian talents at the club.
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Ghanaian Kid Captains Cheslea Youth To FA Cup Glory