Preheat your oven to 220°C (430°F) for conventional ovens—if using a fan-assisted oven, refer to note 2 for adjustments. Generously grease and flour four 3.5-inch (6-ounce) ramekins. If your ramekins are slightly larger or smaller, that’s okay; just adjust the baking time accordingly. In a heatproof bowl, combine the butter and chocolate. Melt them together either over a double boiler or in the microwave, which I prefer for convenience. If microwaving, heat in 20-second intervals, stirring between each until the chocolate is almost completely melted. Then stir continuously to let the residual heat finish melting the chocolate. Once smooth, set it aside. In a large bowl, add the brown sugar, vanilla, salt, whole eggs, and egg yolks. Using a hand mixer or a stand mixer fitted with a whisk attachment, whip the mixture at medium speed for about 5 minutes until it becomes thick and pale. The batter should be thick enough to form ribbons that hold for a few seconds before sinking back in. Sift the all-purpose flour into the egg mixture, then fold in the melted chocolate gently using a spatula until fully combined and smooth. Divide the batter evenly among the ramekins, filling them about three-quarters full. Lightly tap the ramekins on the countertop once to release any air bubbles, then place them on a baking tray. Bake for 11 to 12 minutes, or until the edges are firm but the center still jiggles slightly when touched. Refer to note 3 if you’re concerned about overbaking. Let the cakes cool in their ramekins for about one minute, then carefully invert them onto serving plates or saucers—use a tea towel or oven mitts as the ramekins will be hot. Allow them to rest for about 15 seconds before gently lifting the ramekins off; the cakes should release easily. Finish by dusting with powdered sugar and serve immediately. For extra indulgence, add fresh berries, ice cream, or whipped cream on the side.