01 - Preheat the oven to 425°F. Generously butter four 6-ounce ramekins and dust with cocoa powder or flour, tapping out the excess.
02 - Using a double boiler, melt dark chocolate and butter together, stirring until smooth. Remove from heat and let cool slightly.
03 - In a separate bowl, whisk eggs, egg yolks, and powdered sugar until thick, pale, and slightly foamy, about 2 minutes.
04 - Gently whisk the melted chocolate mixture into the egg mixture until combined, then add vanilla and salt.
05 - Sift flour over the batter and fold in gently until just incorporated, avoiding overmixing.
06 - Divide batter evenly among prepared ramekins, filling each about three-quarters full.
07 - Place ramekins on a baking sheet and bake for 11 to 13 minutes, until edges are set but centers remain soft and slightly jiggly.
08 - Let cakes cool for 1 minute. Carefully run a knife around edges, invert onto plates, and lift off ramekins.
09 - Dust with powdered sugar and serve immediately with fresh berries and vanilla ice cream, if desired.