Add the chopped chocolate, butter, and oil to a large heatproof bowl. Melt gently until smooth using a double boiler or the microwave in short 20-second bursts, stirring between each one. Be careful not to overheat the chocolate.
Once melted, let the chocolate cool slightly so it’s warm, not hot. Add the egg yolks, vanilla, and salt, then whisk until smooth and well combined. Set aside.
In a clean bowl, using an electric hand or stand mixer (see note 1), whip the egg whites on medium speed until foamy (about 30 seconds). Once foamy, gradually add the sugar and continue whipping until stiff peaks form.
To combine, fold one-third of the whipped egg whites into the chocolate mixture using a spatula. This loosens the batter. Gently fold in the remaining egg whites until just combined, taking care not to deflate the mixture.
Divide the batter evenly between the prepared ramekins and smooth the tops if needed.
Place the ramekins in the fridge for 10–15 minutes while you preheat the oven to 375°F (190°C) conventional (see note 2 if using a convection oven with a fan).
Once the oven is ready, bake for 12 minutes, or until the edges are set but the centres still jiggle slightly. This gives you a soft, molten centre. Avoid overbaking, as cracked tops will cause the soufflés to deflate too rapidly.
Once baked, remove from the oven, dust with powdered sugar, and serve immediately. It is normal for soufflés to begin slowly deflating once out of the oven :)