Take a microwavable bowl.Add the chocolate and butter to it.
Microwave in 30 second bursts on high,6 times.Take the bowl out and mix with the spatula between each burst.If after 6 times its not melted completely repeat the process 1-2 more times.
Once its melted and mixed, set aside to cool.
Separate the eggs-yellows in one bowl and white in another larger bowl.I use my hands to do it.
Whip the egg whites till its foamy and soft peaks form.Add the granulated sugar in increments and keep whipping till the egg whites form hard stiff peaks and looks glossy.
Check the temperature of the chocolate-butter mix, once its not very hot(you can touch it with your finger) add the egg yolks and mix.Sprinkle the salt.
Take about 1/4 of the stiff egg white and mix with the chocolate butter and egg yolk mixture till it lightens in texture and weight.This is done to make it easier to fold the rest of the egg whites.
Now using the Cut and Fold method mix the rest of the egg whites in 2 batches.Take care not to mix in the traditional way otherwise all the air whipped into the egg whites will get away.
Once the chocolate and egg white mixture is incorporated(you will still see some white streaks) pour it in the prepared pan.
Bake for 40 minutes till the cake pulls away from the sides of the pan and the middle is set and not wobbly.