Preheat oven to 220C/430F (conventional - see note 2 if using a fan function), and generously grease and flour four 3.5inch (6 ounce) ramekins. If your ramekins are a little bigger or smaller it's fine, you may just need to adjust the cooking time accordingly.
In a heat proof bowl add in butter and chocolate. Melt over a double boiler or in the microwave (which is what I prefer for ease). If using a microwave, melt for 20 second bursts at a time, mixing in between each burst until the chocolate is almost melted. Once at this stage, continue to stir, allowing the residual heat to melt the remaining chocolate. Once smooth, set aside for now.
In a large bowl add in brown sugar, vanilla, salt, whole eggs and egg yolks. Using a hand or stand mixer on a medium speed (use the whisk attachment if using a stand mixer), whip for 5 minutes until the mixture is thick and pale. You should be able to lift the batter up and make ribbons with it, that sit at the top of the batter for a few seconds before being absorbed back in.
To the egg mixture, sift in the all purpose flour and add in the melted chocolate mixture from earlier. Using a spatula, gently fold until well combined and the batter is uniform.
Distribute the batter into the ramekins. The batter should come about 3/4s of the way up the ramekins. Drop the ramekins lightly on the countertop once, and then place them onto a baking tray. Bake for 11-12 minutes, or until the edges are set, but the centre still jiggles when touched. See note 3 if you're worried about overbaking them.
Let them cool in the ramekins for about a minute, before turning them out onto a serving plate/saucer. Use a tea towel or oven mitts to assist you as the ramekins will be hot. Let them rest for about 15 seconds, before gently lifting the ramekins up - they should come off easily.
Finish off by dusting with powdered sugar and serve immediatlely. You can add berries, ice cream or whipped cream on the side too!