Preheat oven to 350°. Lightly grease a 9 in. springform pan.
In a double boiler, melt the chocolate slowly by stirring constantly. Then add the butter in increments into the melted chocolate. This will create a silky smooth mixture - if the butter is added before the chocolate has melted, it will create separation in fatty solids.
Whisk the egg yolks and 1/2 cup of sugar together on medium-high speed until the mixture turns pale yellow and begins ribbon stage.
Slowly add the melted chocolate mixture into the egg yolk mixture and mix thoroughly. Then add the cornstarch and a pinch of salt into the batter until combined. At this point, the batter will be thick and paste-like. Set aside.
In another bowl, create a meringue by whisking together the egg whites with the remaining sugar until medium-stiff peaks form.
Carefully fold the meringue into the chocolate mixture in three increments. You do not want to over mix or you will lose air in the batter.
Pour cake batter into the greased springform pan. Level it out evenly. Then bake at 350° for 30 to 35 minutes. You will know when the cake is finished baking when it feels firm to the touch.