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Preheat oven or OTG to 175°C (350°F) or convection microwave to 160°C. Grease and line an 8x8-inch square metal baking tin with parchment paper.
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Melt chopped dark chocolate and butter together in a heatproof bowl set over a pan of simmering water (double boiler) or in short microwave bursts. Stir until smooth, then let cool to lukewarm.
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In a separate bowl, whisk eggs, castor sugar, and vanilla extract vigorously for 2 to 3 minutes until pale, frothy, and slightly thickened.
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Pour the lukewarm melted chocolate mixture into the whisked eggs and stir until fully combined and glossy.
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Sift all-purpose flour, cocoa powder, and salt into the bowl. Fold gently with a spatula just until dry streaks disappear. Do not overmix.
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Pour batter into the prepared pan, smooth the surface, and bake for 22 to 26 minutes at 175°C (20 to 24 minutes at 160°C in convection microwave).
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Remove when a toothpick inserted into the center emerges with moist crumbs (not wet batter, and not completely clean).
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Cool completely in the pan for at least 1.5 hours. Lift using parchment paper handles, slice into 16 squares with a clean, warm knife, and serve.