Cossu set up three good chances that fellow forward Antonio Di Natale failed to put away in the first half for the world champions, who were without a number of injured regulars.
Another debutant, defender Leonardo Bonucci, was less impressive as coach Marcello Lippi tried out a three-man backline that was not given an awful lot to do by their fellow World Cup finalists.
AC Milan striker Marco Borriello did not see much of the ball before he came off at halftime in his first match for Italy for two years.
Forward Samuel Eto'o, who has struggled with Inter Milan recently, had a quiet game for a Cameroon side that will be looking to do better in South Africa after a disappointing African Nations Cup campaign that ended in the quarter-finals.
"It was a very useful match," Lippi told Rai television. "We saw the lads who I'd called up for the first time. Borriello had played for the national team before but it was the first time under me.
"Bonucci and Cossu played with good personality and authority."
Italy defender Giorgio Chiellini had a goal disallowed for offside in the first half and Di Natale had a couple of shots blocked in the second.
The Azzurri, drawn with Slovakia, New Zealand and Paraguay in World Cup group F, will play two more warm-ups, against Switzerland and probably Mexico, in early June.
But Wednesday's match was Lippi's last chance to size up his options before he names his squad for the finals.
Cameroon, in group E in South Africa with the Netherlands, Denmark and Japan, will play friendlies against Slovakia and Portugal in the run-up to South Africa 2010.
"I'm satisfied with this match because Italy are world champions," Cameroon coach Paul Le Guen told a news conference. "They are a very experienced team with players who play with lots of composure."
Le Guen, having dropped some of his most experienced players for the friendly after the African Nations Cup disappointment, said:
"I wanted to review my squad this evening. We still have a lot of time to work before the World Cup. We have to be more effective in how we attack around Eto'o."
