Preheat your oven to 350 degrees Fahrenheit.
Fill a medium sized pot with water. Choose a bowl large enough to sit on the rim of the pot to create a double boiler.
Put the chocolate and butter in the bowl, and turn the heat to medium-high. Bring the water in the pot to a simmer, and stir the chocolate/butter mixture together until melted.
Separate the eggs, putting the yolks in a medium-sized mixing bowl, and the whites in the bowl of a stand mixer.
Turn the stand mixer to a high speed, and whip the egg whites until stiff.
In the medium bowl, add the granulated sugar to the egg yolks and beat with a hand mixer until the mixture turns foamy.
Pour the egg yolks into the melted chocolate and use a whisk to combine the batter.
Sift the flour, cornstarch, and baking powder into the bowl with the chocolate. Use a rubber spatula or wooden spoon to mix. The batter will be dry and crumbly, this is okay.
Add the whipped egg whites into the chocolate batter and gently fold the egg whites into the chocolate batter using a rubber spatula until fully combined.
Grease a 9” springform pan. Cut out a piece of parchment paper the size of the bottom of the pan, then lay the parchment paper in the pan.
Pour the batter into the pan and bake for 40-45 minutes or until a toothpick comes out clean.