Preheat your oven to 220°C (430°F) if using conventional settings. If you’re using a fan-assisted oven, refer to note 2 for adjustments. Generously grease and lightly flour four ramekins, each measuring approximately 3.5 inches (6 ounces). Ramekins of a slightly different size are acceptable; just modify the baking time accordingly.

Place the butter and chocolate in a heatproof bowl. Melt them together using a double boiler or microwave. If opting for the microwave, heat in 20-second intervals, stirring between each burst, until the chocolate is almost fully melted. The residual heat will finish melting the chocolate as you stir it to a smooth consistency. Set this mixture aside for later use.

In a separate large mixing bowl, combine the brown sugar, salt, vanilla extract, whole eggs, and egg yolks. Using a hand mixer or a stand mixer fitted with a whisk attachment, beat the mixture on medium speed for approximately five minutes. The result should be a pale, thick batter capable of forming ribbons that sit briefly on the surface before sinking back in.

Carefully fold the melted chocolate and butter mixture into the egg mixture, ensuring it is fully incorporated. Gently sift in the all-purpose flour and continue folding with a spatula until the batter achieves a smooth and even texture.

Divide the batter evenly among the prepared ramekins, filling each to about three-quarters full. To eliminate air bubbles, lightly tap the ramekins against the countertop once or twice. Place them on a baking tray for stability and bake in the preheated oven for 11-12 minutes. The edges should appear set, while the centers retain a slight jiggle when touched (see note 3 to avoid overbaking).

Allow the ramekins to cool for about a minute before turning them out onto serving plates. Use a tea towel or oven mitts to handle the ramekins, as they will still be very hot. Let the cakes rest for 15 seconds after being inverted, then carefully lift the ramekins away. They should release effortlessly.

Finish by dusting the tops with powdered sugar. Serve immediately with optional accompaniments like fresh berries, a scoop of ice cream, or a dollop of whipped cream for an indulgent touch.