Preheat an oven to 200℃ / 392 F. Mix flour and cocoa powder and stir well; set aside. Separate the egg yolk from the egg white, and put it in a large bowl. Put the egg whites in another large bowl and sit in a fridge.

Finely chop the milk chocolate and put it in a large bowl. Pour whipping cream into a small pot, and warm it until just before boiling. Pour it over the chopped chocolate. Wait for about 30 seconds, then stir until smooth. Set aside.

Add granulated sugar to the egg yolk and beat well until heavy and whitish. Add the chocolate cream in 3 parts and stir until smooth each time.

Take the egg white out from the fridge. Use an electric hand mixer to beat on high speed. Add granule sugar in two parts when it turns whitish a little, and beat well each time. Beat it until peaks form. The tip should hang down deeply. Beat slowly on low speed for a min to smooth it.

Sift the flour and cocoa powder over the egg yolk and chocolate mixture and stir until smooth.

Add the meringue in 3 parts and use a spatula to fold it into the cake batter. You should add the next meringue before you mix it up thoroughly. Fold it until smooth.

Pour the batter into paper-lined muffin pan cups. Lower the oven to 180℃ / 356 F and bake for 30 minutes. Remove from the oven and let cool as it is for 15 minutes.
It should rise well enough to crack, and the center should slowly go down after baking.

Remove from the pan and cool completely on a rack. Put in a container and cover with cling wrap. Let sit in a fridge overnight. Sprinkle powdered sugar on the top. It's all done! Chewy when cooled, soft and fluffy when it leaves at room temperature.