Adjust oven rack to center position and heat oven to 400 degrees.

Generously butter and flour (or use cocoa powder) eight 6-ounce ramekins or Pyrex custard/baking cups; tap out excess flour and position ramekins on shallow roasting pan, jelly roll pan, or baking sheet.

Meanwhile, melt 8 tablespoons butter and chocolate in medium heatproof bowl set over a pan of almost simmering water, stirring once or twice, until smooth; remove from heat.

Beat eggs, yolk, vanilla, salt, and sugar at highest speed in bowl of a standing mixer fitted with whisk attachment until volume nearly triples, color is very light, and mixture drops from beaters in a smooth, thick stream, about 5 minutes.

Scrape egg mixture over melted chocolate and butter; sprinkle flour over egg mixture.

Gently fold egg and flour into chocolate until mixture is uniformly colored.

Ladle or pour batter into prepared ramekins. (Can be covered lightly with plastic wrap and refrigerated up to eight hours. Return to room temperature for 30 minutes before baking.)

Bake until cakes have puffed about ½-inch above rims of ramekins, have a thin crust on top, and jiggle slightly at center when ramekins are shaken very gently, 12 to 13 minutes.

Run a paring knife around inside edges of ramekins to loosen cakes and invert onto serving plates; cool for 1 minute and lift off ramekins.

Sieve light sprinkling of confectioners' sugar or cocoa powder over cakes to decorate, if desired, and serve immediately with optional whipped cream.