Ok, so dusting with cocoa powder is easier to do, and has a longer shelf life, but its also messier and the results are more limited. You would just toss some cocoa powder in a bag with the puffed rice (if you're puffing your own, best done before cooling), and shake it up. The surface of the puffed rice would hold a thin layer of cocoa powder, and since both ingredients are shelf stable, they should last a long time.
Downsides are, while cocoa powder has an intense taste, it isn't sweet without diluting it with sugar, how much you can add is strictly limited by the surface texture of your puffed rice (and sugar will compete for space), and milk will lift it right off your rice (though puffed rice in chocolate milk could be nice, too). A fine application of oil, water, or milk could help the powder adhere better, but at the sacrifice of some crispiness.
A thin chocolate coating, on the other hand, gives a more familiar product, is less messy, and more variable, but it also has a more limited shelf life (oil can go rancid, the chocolate can bloom, it is simply moister even if the moistness is cocoa butter) and is a bit harder to do. You would gently melt your plain chocolate (with a double boiler, preferably), and mix into a reasonably large quantity of puffed rice, then keep mixing as it cools so that the rice will separate or at least make small clumps, then spread it out (say, on a cookie sheet) to finish cooling and drying. Further dehydrating with heat, sunshine, or air circulation, can be done to preference.
Since chocolate is oil-based, your rice should remain fairly crisp, unlike water based sauces. You may find it easier to make larger clumps with this method, depending on ratio of chocolate and amount of space to spread out and dry on, or make a solid block of chocolate and puffed rice and cut to cereal size, or to coat a batch quite heavily and mix it into a plain batch, or drizzle the chocolate into your cereal rather than try to mix to an even coating, to tweak your preferred ratio, texture, and appearance.
Another thing you might try, depending on your puffed rice product, is comfiting, or sugar-coating, your puffed rice. You would want to create a fairly dry sugar syrup, since moisture will soften your rice, mix in some chocolate (cocoa powder would be best), and drizzle the hot mix into the puffed rice a little (tbs or less) at a time, mixing until dry to form a veerry thin layer, and periodically setting aside to cool and dry more before proceeding with more layers. This method will give many, many thin coatings of chocolate syrup to your puffed rice, and a fair amount of control how thick the coating will be, though it takes a lot more time and patience. Additionally, some extra time drying out and possibly other measures (fan, dehydrator, sunshine, warm oven) can be used to drive off any extra moisture once the rice is coated to your liking, to maximize crispiness and shelf life.
Or, you might try making a rice cereal, if you want - making a chocolate rice dough, probably chilling and grating to make right-sized pellets, and frying them to make your cereal. I would suggest you find a more precise recipe, I can make a basic guess how such cereals are made but I haven't tried to make any.