Steps

1. Preheat oven to 170℃ (340°F).
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2. Measure 10g of granulated sugar for the egg yolk and 25g of granulated sugar for the egg white separately.
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3. Separate 1 egg yolk and 1 egg white. Place the yolk in a small bowl and the white in a large bowl.
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4. Measure 12g of cake flour and 20g of cocoa powder into the same container and mix well with a whisk.
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5. In a large heatproof bowl, combine 40g of unsalted butter, finely chopped 50g chocolate bar, and 40g heavy cream.
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6. Lightly beat 1 egg yolk in the small bowl, add 10g granulated sugar, and mix until pale and creamy.
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7. Heat the chocolate, butter, and cream mixture in the heatproof bowl in a 600W microwave for 1 minute 30 seconds. Stir the bowl to combine, then heat again. (This is the key!) Since heavy cream is included, it can splatter, so heat while watching carefully.
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8. Gradually mix the melted chocolate mixture from the center until completely smooth.
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9. Add the pale and creamy egg yolk and sugar mixture to the melted chocolate and mix well.
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10. In the large bowl, add 1 egg white and 25g granulated sugar all at once. Whip with a hand mixer or whisk until stiff peaks form, or until the peaks just slightly bend over. (This is the key!) The amount of sugar is generous for the amount of egg white, which helps create a meringue that won't become dry and crumbly. Whip until you feel it can't get any stiffer.
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11. Line muffin tins with parchment paper and prepare a piping bag.
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12. Whisk the meringue once with a whisk, then add 1/3 of the meringue to the chocolate mixture and mix briefly with a whisk.
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13. Add the mixed 12g cake flour and 20g cocoa powder, sifting them in, and mix with a rubber spatula until no dry flour remains.
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14. Add the remaining meringue to the chocolate mixture and fold with a rubber spatula until no lumps of meringue remain.
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15. Transfer the batter to a piping bag and pipe 6 equal portions into the muffin tins. Tap them lightly from above to release air.
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16. Bake in the preheated 170℃ (340°F) oven for 10 to 20 minutes. (This is the key!) The surface should have large cracks, and a skewer inserted should come out clean with no raw batter attached.
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17. Once baked, let cool until you can touch it, then transfer to a wire rack to cool completely.
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18. For the finish, dust generously with powdered sugar (non-melting).
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