[For Preparation]
Separate the egg yolk from the egg white. Put the egg yolk in a large bowl. Put the egg white in another large bowl and let it sit in a freezer. Melt chocolate in a microwave (600W for about one and a half minutes). Warm milk in a microwave until body temperature (600W for about 30 seconds).
Preheat an oven to 210℃ / 410 F. Prepare plenty of boiling water.

With stirring, pour oil little by little over the egg yolk. Stir for another 1 minute until smooth after adding all.
Add the melted chocolate and stir well until smooth.

With stirring, pour the milk little by little over. Stir for another 30 seconds after adding all.
Sift cake flour, bread flour, and cocoa powder over. Stir it until smooth, and then beat speedy 40 times.

Add 1 Tbsp rum (or water) and stir well until smooth.
Fill a deep-sided container with plenty of hot water at 50℃ / 122 F, put the bowl in it, and keep the chocolate batter warm. If you mix cold water and boiling water in the same ratio, it reaches about 50℃ / 122 F.

Make meringue.
Take the egg white out from the freezer. Use an electric hand mixer to beat on high speed. When it turns whitish, add half the granulated sugar.

Beat it another 1 minute until heavy, and add the other half of sugar and beat it until stiff peaks form. Beat until lightly tilted tips form. Beat slowly on low speed for a min to smooth it.

Remove the chocolate batter from the hot water, add a scoopful of meringue to it, and use a whisk to stir well until smooth.

Add the remaining meringue in 3 batches. The chocolate batter breaks the bubbles of meringue, so gently mix it as quickly as possible.
Use a whisk to press the meringue lightly, and then mix it while scooping up the batter until you don’t see the big chunk of meringue left. You should add the next meringue before you mix it up thoroughly.

After adding all, when the batter color starts to become uniform, scoop up the batter from the edge or bottom 10-20 times until smooth.
Pour into an ungreased tube pan until it's 80 percent full.

Stir lightly with a bamboo skewer. Hold down the top of the tube and drop it lightly 2 times to remove large bubbles. Lower the oven to 180℃ / 356 F and bake for 35-40 minutes.
Pour the remaining batter into 1 or 2 muffin paper cups and let them sit in a freezer.

Upside it down immediately after baking and cool for 4 hours or more.
Preheat the oven to 200℃ / 392 F. Lower the oven to 180℃ / 356 F and bake the muffin paper cups for 25 minutes.

Remove the cake from the pan. Use your hands to press gently the edge and center (around the tube) to press down the cake until half the height.

Upside down, hold down the bottom plate and lift the side to push the cake out. Next, gently press the outside of the cake to peel it off from the bottom plate.
Peel off the paper from the muffin cake.

For Topping
Add granulated sugar to the whipping cream. Whip it until soft peaks form while cooling in ice water.
Transfer it to a piping bag fitted with a star tip. Fill the hole in the middle with whipped cream. Pipe a swirl on the top. Sprinkle cocoa powder over.

Poke a hole into the center of each muffin. Fill the holes with whipped cream. Pipe a swirl on the top. Sprinkle cocoa powder over.