The 18-year-old struck twice, while Sergi Roberto, Gerard Pique, Marc Bartra and Sandro Ramirez all scored one goal apiece during a facile fixture for Luis Enrique's men
The game was over by the break, with Munir El Haddadi helping himself to a double and Sergi Roberto and Gerard Pique bagging one goal apiece.
Marc Bartra added a fifth just after the break and Sandro Ramirez put the seal on a resounding preseason win by netting with eight minutes to go.
Munir's performance will really have caught Luis Enrique's eye as the coach looks to mastermind an upturn in fortunes for Barca following last season's disappointments.
Barca meets Leon in its final preseason friendly on Aug. 18 before kicking off its Liga campaign with a home tie against Elche on Aug. 24.
Starting with an XI comprised entirely of La Masia graduates, Barca made a roaring start, El Haddadi volleying across HJK goalkeeper Michael Tornes after a superb scooped pass from Andres Iniesta.
As the hosts struggled in front of a healthy Sonera Stadium crowd, El Haddadi again showed his class, releasing Roberto with a neat one-two and seeing the midfielder calmly slot home.
El Haddadi headed Rafinha's cross in via Tornes' right-hand post for his second of the afternoon and when Pedro worked the ball into Pique to tuck home, Luis Enrique's side looked on for a big score.
The remainder of the first half passed by sedately, but Barca emerged revitalized after the break and Bartra headed halftime substitute Ivan Rakitic's free kick beyond replacement HJK keeper Carljohan Eriksson.
Pedro was unfortunate to see his dipping 30-yard volley clip the crossbar on its way over as Barca sought a sixth.
It eventually arrived in the 81st minute as Sandro latched onto Rakitic's measured through ball and lashed a fine strike across Eriksson.
Sandro came close soon after, but his shot drifted wide as Barca came through unscathed and added another impressive showing to its preseason preparations.