Grease a springform pan with butter and dust with flour. This helps the cake release more easily from the pan. Set aside for later.

Melt butter, chocolate, and powdered sugar in a saucepan over low heat. Stir regularly to prevent it from burning.

Pour the chocolate mixture into a large bowl and stir in the egg yolks until you have a smooth mixture.

Whisk the egg whites until stiff; this takes some time, so it's best to use a stand mixer.

Gently fold the whipped egg whites into the chocolate mixture. Do this carefully so you don't deflate the egg whites.

Pour half of the mixture into the springform pan and bake at 170°C in a regular oven for 30 minutes. Set the rest of the chocolate mixture aside in the fridge.

Take the cake out of the oven; it will collapse slightly.

Pour the remaining chocolate mixture over the cake and refrigerate for at least 2 hours, preferably overnight.

Before serving, remove the ring of the springform pan. Run a warm knife around the edges of the cake before loosening the ring. This helps the ring release without taking any cake with it.

This is when you'll see if the chocolate was of good enough quality. If the chocolate was not good, the cake will run out onto the table.

Dust the cake with cocoa powder (or powdered sugar as seen in the picture) and serve with berries or vanilla ice cream.