01 - Preheat the oven to 425°F (220°C). Grease four 6-ounce ramekins with butter, then lightly dust with flour, tapping out the excess.
02 - Melt the dark chocolate and butter together in a heatproof bowl over simmering water or in the microwave, stirring until smooth. Remove from heat and allow to cool slightly.
03 - Whisk eggs, egg yolks, and granulated sugar in a separate bowl until pale and slightly thickened, about 2 minutes.
04 - Fold the melted chocolate mixture into the egg mixture gently until combined.
05 - Sift the all-purpose flour and fine sea salt into the chocolate mixture and carefully fold until just incorporated.
06 - Divide the batter evenly among the prepared ramekins.
07 - Place ramekins on a baking tray and bake for 10–12 minutes, until edges are set but centers remain soft and slightly jiggly.
08 - Remove cakes from the oven, let rest for 1 minute, then run a knife around the edges and invert each cake onto a serving plate.
09 - Dust with cocoa powder or powdered sugar if desired. Serve immediately with a scoop of vanilla ice cream and garnish with fresh berries or mint.