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1
Heat the oven to 170 °C. Butter the pan generously and dust it with almond meal, tapping out the excess. Set aside.
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2
Spread the ground almonds on a baking tray and toast in the oven for 6–8 minutes, stirring once, until they are pale gold and fragrant. Remove and allow to cool completely.
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3
Melt the chocolate and butter together over a bain-marie, stirring until the mixture is glossy and fully combined. Stir in the espresso. Remove from the heat and allow to cool until just warm to the touch.
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4
Beat the egg yolks with the sugar until the mixture is pale and thick, then stir it into the cooled chocolate mixture. Fold in the toasted ground almonds.
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5
This is where the cake lives or dies: beat the egg whites to stiff, glossy peaks, then fold them into the chocolate-almond batter in three additions, working with a light hand so as not to deflate the whites. The finished batter should be airy and uniform, with no visible streaks of white.
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6
Pour the batter into the prepared pan and bake at 170 °C until the exterior is set and shows slight cracking at the surface, but a skewer inserted into the centre meets moist, almost truffle-like resistance — not liquid, not dry. The centre should yield, not spring back.
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7
Remove from the oven and allow the cake to cool completely in the pan. It will sink at the centre as it cools; this is correct and expected. Unmold only once fully cold. Dust generously with powdered sugar over the characteristic crater before serving.