Preheat oven to 350° F / 180° C (NOT convection).
Cut the chocolate into small chunks (if not using chocolate chips) and heat in the microwave with the butter for 30 second increments, stirring in between until melted. Allow to cool down for a bit.
140 grams bittersweet chocolate, 75 grams butter
Separate the eggs and beat egg whites with a pinch of salt until stiff. Beat egg yolks with the sugar until foamy.
6 eggs, 180 grams granulated sugar
Whisk the cooled down, melted chocolate in with the egg yolk sugar mixture.
Mix together flour, cornstarch and baking powder and sift over the chocolate-yolk mixture.
100 grams all-purpose flour, 50 grams cornstarch, 8 grams baking powder
Add beaten egg whites on top and carefully fold into the batter using a whisk or rubber spatula until just combined.
Prep a springform by rubbing on butter and placing parchment on the bottom. Clip the ring around, then dust the sides with flour. If you don't have a springform, butter a 10 inch cake pan and cut out a parchment paper circle to fit the bottom, then dust the sides with flour.
Fill batter into the spring form.
Bake for 40 to 45 minutes (internal temperature taken in the middle should read 200° F or 94° C). Remove from the oven and cover with a clean linen towel (to prevent it from drying out) and allow to cool for 5 minutes before removing from the pan. Place onto a wire rack, cover with your towel and allow to cool completely.
Cut cake into 3 even layers using a large serrated knife.