Prep: Heat oven to 425F (220C). Generously butter four 6-oz ramekins and dust with cocoa powder. Place on a baking sheet.
Melt: In a heatproof bowl over simmering water, melt the chocolate and butter together, stirring until smooth. Remove from heat and cool slightly.
Whisk: In a large bowl, whisk the eggs, yolks, and sugar until pale, thick, and ribbons fall from the whisk, about 3 minutes.
Combine: Pour the warm chocolate into the egg mixture and fold gently. Add vanilla and salt. Sift in the flour and fold until just combined.
Fill: Divide batter among prepared ramekins.
Bake: Bake 11 to 12 minutes, until edges are set but centers still jiggle.
Serve: Rest 1 minute, run a knife around edges, invert onto plates. Serve immediately with ice cream.