The partnership with the Rainforest Alliance started last year when the Unilever brand used certified cocoa for its Magnum Ghana and Magnum Ecuador products.
The move means that all Magnum products will carry the Rainforest Alliance logo.
To reach the 2015 goal, the aim is for 60% of the cocoa that Magnum uses across its ice cream portfolio to come from sustainable sources by the end of 2012.
By 2015, the brand will be helping approximately 40,000 farmers and their families.
Mick Van Ettinger, Magnum senior vice president, said: "For us, this is as much about making a difference at origin by helping farmers and improving their livelihood and environment, as it is delivering pleasure.
"And through this collaboration, we want to engage a new audience of pleasure seekers worldwide."
Edward Millard, director of sustainable landscapes at the Rainforest Alliance, said: "With consumer demand for ethically-sourced products increasing all the time and the green frog seal gaining relevance and influence among consumers, we are delighted to extend our collaboration with one of the world’s leading ice cream brands."
Magnum is using its Facebook page to promote the tie-up.