I didn´t figure out how to do it but I´ve found a video with the explanation in internet and is quite interesting and I´ve added it below.
Step one: rub the chocolate all over the bottom of the can.
Step two: remove the chocolate with a towel, cloth or paper rubbing and polishing.
Step three: repeat the process until the bottom of the can shines. The can it´s polished enough when it shines and reflects like a mirror.
Step four: find a direct angle with the bottom of the can that reflects the sun´s rays.
Step five: place your tender. Try leaves, sticks or paper.
Step six: reflect the concentrated rays to the tender.
Step seven: hold the beam on the tender until you see smoke.