Preheat the oven to 425°F (220°C)
Grease 4 ramekins with butter
Dust the ramekins lightly with cocoa powder
Melt 1/2 cup butter and 4 oz dark chocolate together
Stir until smooth
In a bowl, whisk 2 eggs, 2 egg yolks, 1/4 cup sugar, and 1 teaspoon vanilla extract
Add the melted chocolate mixture to the egg mixture
Fold in 1/4 cup all-purpose flour and a pinch of salt
Divide the batter evenly among the ramekins
Place the ramekins on a baking sheet
Bake for 10 to 12 minutes
Remove when the edges are set and the centers are soft
Let rest for 1 minute
Invert onto plates
Serve immediately