Chop half of the chocolate and place it in a pot with the condensed milk and evaporated milk.
Cook at low heat (it should not break into a boil) stirring constantly so it doesn't burn.
Let it cook until you have a really thick mix: you'll be able to clearly see the base of the pot for a while before the mix covers it back again.
Transfer the mix into a freezer-friendly container. Try to use a large one so it cools down faster. Wait for it to cool down completely and then freeze covered at least 4 hours or a maximum of 3 weeks. It will not get too hard as it has too much fat and sugar but it will go nice and firm so that you can form the truffles.
While the mix is cooling, make the grated chocolate swirls to cover them. Put the remaining chocolate in the microwave in 5-second intervals until it's barely soft. You can try grating it in between heatings to see if it's soft enough so that you get swirls instead of small and hard chocolate chunks.
Take them to the fridge for at least 30 min or for as long as the truffles are in the freezer. The fact that they're cold will make them easier to work with.
Spread a couple of drops of vegetable oil on the palm of your hands and on a teaspoon.
Use the teaspoon to scoop some truffle mix and then shape it into a ball between your hands.
Pass it through the grated chocolate swirls. If truffles begin to melt, return the mix to the freezer for another 30 minutes and then continue.
Store the truffles in the fridge, especially during summer.