01 - Preheat oven to 425°F. Generously butter four 6-ounce ramekins and dust with cocoa powder or flour, tapping out the excess.
02 - In a heatproof bowl over simmering water, melt chocolate and butter together, stirring until smooth. Remove from heat and cool slightly.
03 - Whisk together eggs, egg yolks, and powdered sugar until thick, pale, and slightly foamy, about 2 minutes.
04 - Gently whisk melted chocolate mixture into the egg mixture until combined. Add vanilla extract and salt.
05 - Sift flour over batter and fold in gently until just combined; avoid overmixing.
06 - Divide batter evenly among 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 - Cool cakes for 1 minute. Run a knife around edges, invert onto plates, and remove ramekins. Dust with powdered sugar and serve immediately with berries and vanilla ice cream if desired.