Preheat the oven to 325°F.

Spray four 4 oz. ramekins thoroughly with nonstick cooking spray and transfer to a baking sheet. Set aside.

Place the chopped chocolate and butter in a large microwave-safe mixing bowl. Microwave on high in 30-second increments, stirring between each, until the chocolate is fully melted and smooth. Allow the mixture to cool at room temperature for at least 5 minutes.

Add the sugar, eggs, egg yolks, and vanilla extract to the chocolate mixture. Whisk gently until the eggs are fully combined and the mixture is smooth. Sift the flour and salt into the bowl, and continue whisking until just combined.

Divide the batter evenly between the prepared ramekins.

Bake the cakes on middle rack of oven until the edges are set, and a slightly darker, quarter- to half dollar-sized wobbly spot remains in the center (when poked with a knife tip, the center should be pudding-like), 9-15 minutes.

Check to see that cakes are done. Remove from oven or add time as needed.

Allow the cakes to cool in the ramekins for 1-2 minutes. Protecting hands, run a knife around the edges of each cake and invert onto serving plates. Serve immediately, while still very warm.