Preheat oven to 425°F (220°C). Generously butter four 6-ounce ramekins and place on a baking sheet.
In a microwave-safe bowl, combine chocolate and butter. Microwave in 30-second intervals, stirring between each, until melted and smooth.
Let the chocolate mixture cool slightly, then whisk in flour, powdered sugar, and salt until just combined.
Add the eggs, egg yolks, and vanilla, whisking gently until just incorporated. The mixture should be smooth.
Divide the batter evenly among the prepared ramekins. They should be about 3/4 full.
Bake for 12-14 minutes until the edges are firm but the centers are still soft. The tops should be set but slightly jiggly in the middle.
Let the cakes cool in the ramekins for exactly 1 minute after removing from the oven.
Run a knife around the edges of each cake to loosen, then carefully invert onto serving plates.
Dust with powdered sugar and serve immediately with vanilla ice cream and berries if desired.