Bryan Mbeumo scored a late penalty to send Thomas Frank’s side up to ninth in the Premier League after a dramatic comeback against Watford.
Visiting goalkeeper Daniel Bachmann tipped over a Shandon Baptiste shot in the fourth minute and then kept out a Mbuemo effort before Watford sprung to life midway through the first half.
Joshua King hit the post for the visitors in the 24th minute and Dennis fired them ahead less than 60 seconds later, powering in a thumping header from a Tom Cleverley corner.
Brentford pressed after the break and Bachmann produced another fine save in the 74th minute to palm away a stinging shot by Christian Noergaard before King had an effort smothered at the other end by home keeper Alvaro Fernandez.
But the home side drew level in the 84th minute as Jansson glanced in a close-range header before Mbeumo turned the match with a penalty deep into stoppage time after William Troost-Ekong fouled substitute Saman Ghoddos.
