Grease four ramekins with butter, including bottom and sides; set aside.
In a medium-size bowl, combine chocolate and unsalted butter. Place over a pot of hot water to create a bain-marie and stir until chocolate and butter are melted.
In a separate bowl, whisk the eggs together with the sugar until thick, then stir in the flour and mix well. Pour the melted chocolate slowly into the egg mixture and mix well.
Divide the batter into ramekins and place them in refrigerator to chill for at least 30 minutes.
Select Bake on the Vortex and set the temperature to 180°C and time to 13 minutes (for a gooey centre) or 14 minutes (for a firmer centre). When display indicates Add Food, place the ramekins inside the cooking chamber.
Bake for time, or until the tops have formed a crust (check with a couple of minutes to go)
Once ready, remove the cooking chamber tray form the vortex and leave the ramekins to rest for 5 minutes.
Use a small knife to loosen the cakes from the ramekin and turn them over onto a serving plate (careful as the ramekins will be very hot!)
Serve with a sprinkle of cocoa powder.