Preheat the oven to 170C/338F. Grease and line a 24cm springform tin. Place it on an oven tray – this will help make sure you can evenly lower it in and out of the oven.
Place the butter and chocolate in a large heatproof bowl over a small pot 1/4 filled with water. Turn the heat to medium and allow the steam to melt the chocolate and butter, stirring intermittently until melted and combined. Set aside.
Separate the egg yolks and whites in super clean bowls, ensuring no yolk gets into the whites. Use the whisk attachment of your mixer to whip the egg whites until frothy, then add the sugar. Whip for 3-5 minutes or until the bowl can easily be inverted upside down without slippage.
Add the egg yolks, cocoa powder, salt and vanilla bean extract to the melted chocolate mixture and stir just to combine.
Use a silicone spatula to spoon about a cup of the whipped egg white into the chocolate mixture. Begin folding to combine. Once that first bit is folded in, add more in 2-3 batches until you have folded in all the whipped egg white.
The mixture should be fluffy, voluminous and a consistent medium chocolate brown colour. There should be no streaks of egg white or of melted chocolate. Be sure to scoop the bottom of the bowl as there is often extra melted chocolate lurking there.
Pour the cake batter into the lined springform tin and place it into the oven. Cook for 30-40 minutes. It can be tricky to determine if a flourless chocolate cake is fully baked but it is also easy to over-bake a flourless chocolate cake. I recommend pulling it out of the oven if in doubt.
Allow the cake to cool completely before serving. For best results, allow it to sit in the fridge overnight. Slice with a hot knife – it is very fudgy.