[Shortbread]
Cut well-cooled unsalted butter into 1 cm (0.4 '') dice and put in a large bowl. Add powdered sugar, bread flour, cake flour, and cocoa.

Use your hands to crush and rub the butter cubes until the mixture turns cocoa-colored and looks like dry crumbs (quickly so that the butter doesn't melt). Press down until it comes together roughly.

Put it out on the work surface lined with plastic wrap, and press it to gather the dough. Use a scraper to divide it in half, stack it, and press it down. Repeat several times.

Flatten it, fold it in half, and press. Repeat ''folding in half and pressing'' 10 times until smooth.
Sandwich the dough between cake films (or plastic wrap), and roll it out to 5 mm / 0.2'' thick. Let it sit in a freezer for 5-10 minutes, leaving the films on.

Use a round cookie cutter, 6 cm / 2.3 inches in diameter, to cut it out. Dust the cookie cutter with bread flour, shaking off the excess flour, then cut.
Re-roll the dough scraps and cut. If the dough turns soft, place it in the freezer for a few minutes before cutting.

First, bake the bottom parts. (Leave the other cookies in the freezer.)
Arrange 10 bottom cookies on a baking sheet lined with a baking mat (parchment paper). Lower the oven to 170℃ / 338 F and bake for 13 minutes.

Remove from the oven and use a small tray to gently press down to flatten the cookies. (The shortbread dough tends to rise if it's thin.)
Let them cool as it is for 5 to 10 minutes, then transfer them to a rack to cool.

Next, bake the remaining cookies.
Preheat an oven to 190℃ / 374 F. Arrange 10 upper cookies and 10 small round cookies on a baking sheet.
Lower the oven to 170℃ / 338 F and bake for 10 minutes. Remove from the oven and let cool for 5 to 10 minutes, then transfer to a rack to cool.

[Milk Chocolate Ganache]
Cut the chocolate into small pieces (about 1 cm / 0.4'' cubes) and place in a heatproof bowl.
Microwave at 600W for 1 minute and mix roughly. Warm it for another 1 minute and stir until smooth. Be careful not to warm it too much or it will separate.

Warm heavy cream in the microwave at 600W for 30 seconds until around 50°C / 122F.
Add to the chocolate at once, and stir while drawing small circles. Once the center begins to blend, stir the side. Stir until smooth.

Use a whisk to stir for 30 sec. Place the bottom of the bowl in cold water and cool until heavy while stirring slowly with a spatula. It should hold its shape when dropped. Be careful not to get water into the chocolate.
When the mixture is cool enough to leave traces when dropped, transfer it to a piping bag fitted with a round tip.

[Make Sndwich Cookies.]
Turn over the bottom cookies and pipe the ganache in the middle. Cover with the holed cookies, and press gently. Please squeeze out plenty of the ganache so that the upper and bottom cookies stick together. Put the ganache between the remaining small round cookies.
Let it cool in a fridge for about an hour to set the ganache. It's all done!