The 18-year-old Barcelona B forward stepped up for Saturday's friendly and looked at home among his senior team-mates, scoring twice and grabbing an assist.
Having opened the scoring, the Spain Under-19 man turned creator for Sergi Roberto to double Barca's lead inside 10 minutes.
Luis Enrique's side were in rampant form and El Haddadi headed his second goal of the game with 17 minutes gone, with Gerard Pique making it four shortly afterwards.
Further goals followed after the interval as Marc Bartra headed home and Sandro Ramirez capped the performance with a well-taken drive.
Yet Munir's performance will have caught coach Luis Enrique's eye as he looks to mastermind an upturn in fortunes for Barca following last season's disappointments.
Barca meet Leon in their final pre-season friendly a week on Monday before kicking off their Liga campaign with a home tie against Elche.
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 revitalised after the break and Bartra headed half-time 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 throughball 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 their pre-season preparations.
