Easy flourless chocolate cake

Method

1. Heat the oven to 180°C/160° fan/gas 4. Gently melt the chocolate and butter in a heavy-based saucepan, then stir in 100g of the sugar. Leave to cool slightly, then beat in the egg yolks and vanilla using a wooden spoon. Pour into a large mixing bowl.
2. In a large, spotlessly clean bowl whisk the egg whites with an electric mixer until foamy, add the cream of tartar, then whisk until stiff peaks form when the beaters are removed. Whisk in the remaining sugar, 1 tbsp at a time, until thick and glossy.
3. Mix a spoonful of egg white into the choc mixture to loosen it, then fold in the remaining whites in 2-3 additions. Using a large metal spoon and 
a figure-of-eight motion, scrape the sides of the bowl, then fold the mix over on top of itself, giving the bowl a quarter-turn each time. Scrape the bottom of the bowl too after each turn to incorporate all the mixture.
4. Spoon the batter into the prepared tin, then bake for 40 minutes or until risen and the top starts to crack. Cool the cake in the tin. Serve warm or at room temperature with ice cream.