Preheat oven to 425°F (220°C). Grease and flour two 6-ounce ramekins.

In a double boiler or heatproof bowl set over simmering water (or in 30-second bursts in the microwave), melt the butter and chocolate together, stirring until smooth. Remove from heat.

Whisk in the granulated sugar until well combined.

Whisk in the eggs and egg yolks one at a time, mixing well after each addition.

Gently fold in the flour and salt until just combined. Do not overmix!

Divide the batter evenly between the prepared ramekins.

Bake for 12-15 minutes, or until the edges are set and the center is still slightly jiggly.

Let the ramekins cool on a wire rack for 1-2 minutes.

Run a knife around the edges of the cake and invert each ramekin onto a plate. Dust with powdered sugar, if desired, and serve immediately.