Finely chop the chocolate and put it in the top of a double boiler or what we call a “bain-marie,” a stainless steel or ceramic bowl that sits over a bigger pot of hot boiling water so that the steam or indirect heat melts the chocolate.
When the chocolate is completely melted, add the pieces of butter and stir carefully with a wooden spoon until a smooth liquid form without any clumps;
Remove the pot from the bain-marie and let it cool down while stirring occasionally;
In the meantime, crack the eggs and carefully separate the yolks from the whites; put them into two big mixing bowls. Make sure there aren’t any yolks in the whites; otherwise, they won’t whip up properly;
Add the same quantity of sugar in both bowls;
Beat the yolks and sugar with an electric whisk for about 3-4 minutes until the mixture doubles in volume;
Pour the melted chocolate into the whipped yolks;
Mix with the electric mixer at low speed or with a hand whisk for a few seconds until even and smooth;
Add lukewarm milk, a little at a time;
Whip the whites with the sugar and a pinch of salt until stiff peaks form. To test this, turn the bowl upside down (they won’t fall out if beaten enough);
When they are white and compact, fold them into the chocolate mixture a little at a time with upward movements in order to not deflate the whites.
Move the egg whites only as much as necessary; don’t stir;
Transfer the mixture into a buttered, floured springform pan (or you may line it with parchment paper); lightly smooth over the surface with a spatula;
Pre-heat the static oven to 350°F (176°C) and bake for 35-40 minutes. Only check its doneness a few minutes before the cooking time is over;
During baking, the cake will appear to rise; if it turns golden brown, take it out of the oven fast and put in on a lower rack;
When the crispy crust starts to form on the surface, after about30-35 minutes, do the toothpick test (don’t open the oven before that). Remember that tenerina cake is a very moist cake, so if it comes out moist, that’s OK! The surface should be dry; check a few parts of the cake;
If it seems too gooey, bake it a few minutes more;
When the surface cracks (it should look like a cracked meringue), your cake is ready. Take it out of the oven and let it cool down completely. Garnish it with plenty of powdered sugar and/or unsweetened cocoa powder. A combination of the two will result in a burst of chocolate.