Grease the pan with unsalted butter. Line the pan with parchment paper. Set aside.

Cut the unsalted butter into thin or small pieces, and put them in a bowl. Add chocolate (small pieces) into the bowl with the butter.

Put the cake flour and cocoa powder into a bowl. Mix well while crushing lumps; set aside. Preheat an oven to 200℃ / 392F.

Separate egg yolks from egg whites. (4 egg yolks, 5 egg whites) Put the egg white in a large bowl. Let the egg white sit in a fridge.

Add granulated sugar to the egg yolks. Mix it for 2 mins until whitish and heavy. Set aside.

Pour water into a small pot. Bring the water to a boil and then turn off the heat. Place the bowl with chocolate and butter on the top of the pot. Place the bowl so as not to touch the hot water directly (to prevent separating). Wait for 30 sec.

Melt the chocolate and butter until smooth. Warm the heavy cream until it comes out steam lightly. (until around 50℃ / 122F), microwave it at 500W for 40-60 sec.

Add the warmed heavy cream to the chocolate in 4-5 parts. Mix well until combined each time. If the cream is too hot, the chocolate may separate, so be careful. Takedown the bowl. Takedown the pot too, and set aside the pot for later use.

Mix the egg yolk mixture lightly. Add half of the melted chocolate. Mix it with a whisk until combined.

Add the other half. Mix it until glossy for around 1 min. Place it on the pot to prevent it from getting cold.

Take the egg white out from the fridge. Beat it lightly until watery. Whip it with a hand mixer on high speed until whitish and fluffy.

Add half of the granulated sugar. Whip it on high speed until heavy. Add the remaining sugar. Whip it on high speed until the peak hangs down deeply.

Whip slowly on low speed for a min to smooth it.

Mix the chocolate mixture lightly. Add the meringue in 4 parts. Stir well with a whisk until combined at 1st addition.

And then sift half of the dry ingredients over. Stir well until smooth.

Meringue; 2nd addition, stir roughly until marble-ish.

Sift the remaining dry ingredients over. Stir well until smooth.

Meringue; 3rd addition, mix the top lightly, and scoop the batter up and let it slip through your whisk; repeat scooping. Continue to scoop until almost combined.

Meringue; 4th addition, mix in the same way as the 3rd addition until combined. And then use a spatula to scrape the batter on the inner side of the bowl, and fold it.

Pour the batter into the pan. Drop the pan lightly to remove big bubbles. Bake it at 180℃ / 356 F for 40-45 mins until the top of your cake crack.

Drop the freshly baked cake lightly about 2 times. Let it cool as it is for about 4 hours. It'll slowly go down.

Remove the cake from the pan. Peel the paper gently. Use plastic wrap to wrap it double. Let it sit in a fridge for 1-2 days.

Dust the cake with powdered sugar. Top with whipped heavy cream and mint. Done!! This cake has a strong bitter taste on the first day. You should sit in a fridge for 1-2 days to get a nice mild flavor. The second day is my favorite.