[Puff Pastry]
Dice unsalted butter into about 1cm (0.4-inch). Chill all the ingredients for the puff pastry in a fridge for 30 mins.
30 mins later, put butter, bread flour, cake flour, and salt in a bowl. Toss to coat the cubed butter with flour. Crush and rub the butter cubes while mixing.

Use a scraper in the second half to mix while cutting to prevent the butter from melting by the heat from your fingers. Mix it until yellowish and coarse lumps.
(Please cool it in the fridge if the butter seems to melt.)

Put it onto plastic wrap, and press lightly to shape it square. Wrap it with plastic wrap and let it sit in a fridge for 3 hours.

Flour your work surface, dough, and rolling pin. Roll out into a long rectangle of 5-7mm (0.2 to 0.3-inch) thick. Fold the bottom 1/4 of pastry up, then the top 3/4 down. Brush off excess flour. Then fold it in half. Chill in the fridge for 30 minutes.

Place the folded dough on the work surface so it looks like a book about to be opened.
In the same way, roll out, fold, and chill.

[Blind Bake]
Preheat the oven to 200°C / 392 F.
Divide the dough into 6 equals. Roll out each to 2 mm (0.08- inch) thick while coating with bread flour. Use a 10cm / 4 inches ring to cut out.

Gently press into the tart pan. Prick the base with a fork. Repeat with the remaining pieces.

Gather the dough scraps and knead lightly until come together. In the same way, roll it out, and cut it out.
Gently press into the muffin pan. Prick the base with a fork.

Line each cup with aluminum cup cases (paper muffin cases). Fill the inside with tart stones.
Lower the oven to 180°C / 356 and bake for 25 minutes. Remove the tart stones and paper cases.
*If the bottom is not baked (it's moist), bake for another 3-5 minutes after removing the stones and paper cases.

【Chocolate Cake】
Put the chopped chocolate and butter in a bowl and heat it in a microwave (600W for about 1.5 minutes), then mix to melt until smooth. Preheat the oven to 190°C / 374 F.

Put an egg and granulated sugar in a large bowl and mix well.
Fill a deep-sided container with plenty of hot water at 50℃ / 122 F, and put the bowl in it. (If you mix cold water and boiling water in the same ratio, it reaches about 50℃ / 122 F.)
Warm the egg mixture until 35 ℃ / 95 F remove from hot water.

Use an electric mixer to beat on high speed until whitish and heavy. The batter should fall like a ribbon, and hold its shape for a moment. Then beat on low speed for 30 seconds to smoother it.

Sift the cocoa powder into the melted chocolate and stir well until smooth.

Add a scoop of the egg batter to the chocolate and mix well until smooth. Put it back into the cake batter. Use a whisk to mix it until smooth while gently scooping up the batter. Finally, use a spatula to fold the edges batter 2-3 times.

Transfer the batter to a piping bag, and Fill the inside of the puff pastry cups with the batter. Lower the oven to 170℃ / 338 F and bake for 18 minutes. Remove it from the oven, cool it for 15 minutes, and gently remove it from the mold.

The outside is crispy and the inside is a soft chocolate cake. The chocolate cake will harden and chewy if you chill it. Warm it up a little in the microwave if you want it to be soft again.