• Step 1: Preheat your oven to 450°F (230°C). Grease four 6-ounce ramekins or oven-safe custard cups with butter, then dust with a light coating of flour.
• Step 2: In a medium heatproof bowl, melt the 4 tablespoons of butter and the chopped chocolate together by placing the bowl over a saucepan of simmering water, stirring frequently until smooth. Remove the bowl from the heat and let the mixture cool slightly.
• Step 3: In a separate bowl, whisk together the eggs, egg yolks, and sugar until well combined and slightly thickened.
• Step 4: Fold the egg mixture into the slightly cooled chocolate mixture until just combined. Then, gently fold in the 2 tablespoons of all-purpose flour and a pinch of salt until no dry pockets remain.
• Step 5: Divide the batter evenly among the prepared ramekins, filling them about three-quarters full.
• Step 6: Bake for 8 to 10 minutes, or until the edges are firm but the centers are still soft and molten. Be careful not to overbake.
• Step 7: Remove the ramekins from the oven and let them cool for 1 minute. Then, run a knife around the edges and invert the cakes onto individual plates or shallow bowls.
• Step 8: Dust the warm cakes with powdered sugar, if desired, and serve immediately.