Preheat the oven to 425°F (218°C) and grease 4 ramekins with butter. Dust them lightly with flour.

Melt the butter and chocolate together in a heatproof bowl over simmering water or in the microwave. Stir until smooth.

In a separate bowl, whisk together the sugar, eggs, and egg yolks until light and fluffy. Add the melted chocolate mixture and mix until combined.

Fold in the flour and salt until just combined. Divide the batter evenly among the prepared ramekins.

Bake for 10-12 minutes, until the edges are set, but the center is still soft. Let cool for 1 minute, then invert onto plates.

Serve immediately with powdered sugar, ice cream, or fresh berries.