Meanwhile, in another bowl, melt the chocolate, either over a pot of boiling water or in the microwave, if you prefer.
As soon as it is melted, pour the egg yolks into the chocolate, and immediately stir them into it.
Don't let the yolks sit on the hot chocolate without mixing, or they may begin to cook and not smoothly incorporate into it.
Most people will say to gently fold the chocolate into the whites so that they don't fall. That is the correct way of doing things, of course, but I have found that sometimes the chocolate and egg yolk mixture gets too stiff to incorporate it easily. I've found that this mousse turns out well when I use my electric beater to mix it all together, when needed. To prevent the whites from falling too much, do your best to mix as gently and for as little time as possible.
As soon as the ingredients are well combined, pour the chocolate mousse into cups or small bowls. Cover the bowls, and place them in the refrigerator. The consistency improves as the mousse chills, so it's best to make it at least a couple of hours before serving.
You can add a little more cocoa powder if you want a stronger chocolate flavor. You can also add coffee powder or alcohol for different flavors. To make it sweeter you can add sugar or honey. The sweetness also depends on the chocolate that you use, so you can experiment with it to find the best combination for you. I normally make it just as written above: simple yet elegant.
Since part of the eggs stay raw, I use my freshest eggs, just in case. That said, I've made this many times with supermarket eggs, before I had hens myself, and I've never had a problem. While the risk of contracting salmonella from eating raw eggs is very small, it is a risk that you should keep in mind. If you are concerned, it has been brought to my attention that may areas have pasteurized eggs available for use in these sorts of recipes. (I've never seen them here in Spain.)