Butter four 6-oz ramekins thoroughly, then dust with cocoa powder and tap out excess. Place on baking sheet and chill in freezer for 15 minutes. Preheat oven to 425°F.
cocoa powder
Combine bittersweet chocolate and butter in a heatproof bowl over simmering water or microwave in 30-second bursts, stirring between each. Stir until completely smooth and glossy. Whisk in vanilla extract and espresso powder if using.
6 oz bittersweet chocolate, 6 tbsp unsalted butter, 0.5 tsp vanilla extract, 0.5 tsp espresso powder
In a separate bowl, whisk together eggs, egg yolks, granulated sugar, and salt for 2-3 minutes until pale yellow and slightly thickened. Mixture should fall in ribbons when whisk is lifted.
2 large eggs, 2 egg yolks, 0.25 cup granulated sugar, pinch salt
Pour melted chocolate into egg mixture and fold gently with a spatula until combined. Sift flour over the top and fold just until no streaks remain. Stop mixing immediately to avoid overdeveloping gluten.
2 tbsp all-purpose flour
Divide batter evenly among chilled ramekins (about ¾ full). Bake for 12 minutes until edges are set and pull away from sides but centers still jiggle slightly when gently shaken. Cool 1-2 minutes, run a thin knife around edges, then invert onto plates. Dust with powdered sugar or cocoa powder.
powdered sugar, cocoa powder
