Prepare ramekins – Preheat oven to 375°F (190°C). Grease four 6-ounce ramekins with softened butter, then coat with sugar, tapping out excess.
Melt chocolate – In a heatproof bowl over simmering water (double boiler), melt the chocolate and 2 tbsp butter until smooth. Remove from heat and let cool slightly.
Separate eggs – Place yolks in one bowl and whites in another.
Make base – Whisk egg yolks with 1/4 cup sugar and vanilla until pale and creamy. Slowly whisk in melted chocolate mixture.
Whip egg whites – Add a pinch of salt to egg whites and beat with an electric mixer until foamy. Gradually add 1 tbsp sugar and beat until stiff peaks form.
Fold mixture – Gently fold one-third of the egg whites into the chocolate mixture to lighten it. Carefully fold in the remaining egg whites without deflating.
Fill ramekins – Spoon mixture into prepared ramekins, smoothing the tops. Run your thumb around the inside rim for an even rise.
Bake – Place ramekins on a baking sheet and bake for 12–15 minutes, until risen and slightly set but still soft inside.
Serve immediately – Dust with powdered sugar if desired and serve right away for the best texture.