Brush the pan with unsalted butter. Sprinkle granulated sugar onto the pan. Tap the pan lightly to remove excess sugar. Let it sit in a fridge. Sift cake flour and cocoa powder together. Preheat an oven to 150℃ / 302 F.

Separate egg yolks from egg whites. Let the egg white sit in a fridge. Cut unsalted butter and milk chocolate into small pieces and put them in a bowl.

Pour water into a small pot and bring it to a boil and then turn off the heat. Put the bowl (previous butter and chocolate) on the top of the pot so as not to touch the hot water directly. Wait for 30 sec and then melt it until smooth.

Remove the bowl from the pot. Put a container with heavy cream in the pot. Leave it warm in the hot water as it is ; set aside. (You don't need to heat the hot water again.) Add egg yolks one each to the chocolate and mix well each time. Add the heavy cream in 4 parts. Mix well each time.

After adding all the heavy cream, continue to mix for another 6 mins. 2 mins later, it will emulsify and become heavy and glossy. 6 mins later, it will be pale. Mix it until pale like 3rd picture below.

Pour 300 ml boiling water and 300 ml water into a container. And put the chocolate mixture in it to warm. Leave it until you use. Take the egg white out from the fridge and beat it lightly until watery.

Whisk the egg white with a hand mixer on high speed until white and fluffy. Add 1/3 of the granulated sugar and whisk. 30 sec later, add the rest of sugar. Whisk it until very soft peaks form or the top of the peak hangs down. Please be careful not to form stiff peaks. And then whisk it slowly on low speed for a min.

Put the chocolate mixture in a large bowl. Add 1/3 of the meringue and mix until combined.

Sift flours again and put it in the bowl. Mix it slowly with a whisk until combined. After combined, mix 20 times quickly.

Add half of the rest of the meringue and fold it with a spatula until combined. Add the rest of meringue and fold it until combined. Add rum and fold it until combined (about 20 times).

Put the batter into the prepared loaf pan. Flatten it roughly and drop the pan lightly about 10 times. Pour hot water (about 50℃, 122 F) into a baking pan. Place the loaf pan in the hot water and bake it at 150℃ / 302F for 40 mins.

Take the pan out from the hot water gently. Leave it cool for 5 mins. 5 mins later, cover the top of the cake with plastic wrap. Leave it as it is for an hour to make it shrink slowly.

An hour later, wipe the water drops of the plastic wrap and wrap it again. Let it sit in a fridge for 3 hours.

Slide a warmed knife around the edges and remove the cake from the pan. Make the side of the cake smooth with a warmed knife. Sprinkle cocoa powder on the top of cake. It's all done!