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 semisweet chocolate together
Stir until smooth
In a separate bowl, whisk 2 eggs, 2 egg yolks, 1/4 cup sugar, and a pinch of salt
Add 1 teaspoon vanilla extract
Fold the melted chocolate mixture into the egg mixture
Add 1/4 cup all-purpose flour
Stir until just combined
Divide the batter evenly among the ramekins
Place the ramekins on a baking sheet
Bake for 12 to 14 minutes
Remove when the edges are set and the centers are still soft
Let cool for 1 minute
Invert onto plates and serve immediately