Preheat a fan forced oven to 180C / 355F. Spray the bottom of an 8-inch (20cm) round springform pan. Line with baking paper.
Fill a large pot half way with water and bring to a boil. Place a heatproof bowl on top making sure the bottom of the bowl isn’t touching the water. Add the butter and chocolate and stir until melted and combined. Set aside to cool.
Add the egg yolks to a large mixing bowl along with the sugar, rum and vanilla extract. Use an electric hand mixer to beat on high speed until pale and thick, about 5 minutes.
Add the cream and 1/4 of the melted chocolate and whisk through. Add the remaining chocolate and whisk until well combined.
Add the cream of tartar and egg whites to a large, clean metal or glass bowl. Using clean beaters, beat on high speed until it froths and reaches stiff peaks. Add half the egg whites to the chocolate mixture and fold through using a spatula. Then add the remaining egg whites and repeat.
Pour the batter into the cake tin and bake for 25 minutes in the middle rack of your oven.
Once baked and cooled, run a knife around the sides of the cake to loosen it. Dust with cocoa powder and unclip the cake tin. Transfer to a serving plate, slice and serve.