This recipe starts with the undisputed main character of the story: the bitter chocolate! Here the one we use in Emilia Romagna (well, the name says it all). But a good bitter chocolate will work just fine! But, before you remember this at the end of the recipe, when it's too late... turn on the oven, static mode, 180 °C :)
Start finely chopping the bar, so that it will melt easier. Put the chocolate into a pot and make it melt in a water bath (in Italian we say "bagnomaria").
As soon as the chocolate has melted (but before it's hot) add the butter and let it melt, always gently mixing. Switch the heat off now, and let the mix cool down.
While the chocolate mixture gets colder, separate egg whites from yolks, into the bowls. Whisk the egg whites with half sugar until it gets thick and firm.
In the other bowl, whisk the yolks with the remaining half sugar, until it gets fluffy.
Keep on whisking, and slowly add the chocolate mixture into the yolks bowl. Add and whisk until you obtain a smooth paste.
Now the most important step: adding the whisked egg whites to the paste. Do this in 3-4 steps, and do it gently, using a spatula to mix, with a gentle bottom-up movement. This is important!
You are almost there :) Now slowly add the flour while you keep on gently mixing with the same bottom-up movement, until all the flour is incorporated.
Grease the springform pan with some butter and spray it with flour, so that the cake won't stick to it. Pour the mixture into the pan and put it into the static oven, that should have reached the proper temperature (180 °C).
After 25 minutes your tenerina cake is ready and can be removed from the oven. Let it rest and get cool, remove it from the springform pan.....
... and spray its incredible, crusty surface with icing sugar. Enjoy! :)