Preheat oven to 425 degrees F (220 degrees C). Grease six 6-ounce ramekins with butter. In a double boiler, melt the butter and chocolate over low heat, stirring occasionally, until smooth. In a large bowl, whisk together the eggs, egg yolks, sugar, and salt until well blended. Gradually whisk in the melted chocolate mixture until smooth. Fold in the flour until just combined. Divide the batter evenly among the prepared ramekins. Bake for 10-12 minutes, or until the edges are set and the centers are still gooey. Let cool for 1 minute before inverting onto individual plates and serving immediately.