Chocolate angel cake

Method

1. Put the egg whites in the bowl of a stand mixer with the whisk attachment fitted and leave to stand for 30 minutes to ensure the egg whites are at room temperature.
2. Meanwhile, make the ganache. Put the dark and milk chocolate in a heatproof bowl. In a small pan, heat the cream until steaming. Tip the cream over the chocolate before it starts to boil, then set aside for 5 minutes to melt the chocolate. After 5 minutes stir to combine into a smooth ganache. Let the ganache cool for 1-2 hours, stirring every 15 minutes or so.
3. Sift the flour, cocoa powder and 50g of the sugar into a bowl. Then sift again,  twice more, to make sure the ingredients are as fine as possible and set aside.
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4. Heat the oven to 150°C fan/gas 3. Add the cream of tartar to the egg whites and whisk until stiff. Add the remaining 150g sugar one spoonful at a time while continuing to whisk (as you would meringue) until all combined, shiny and stiff. Beat in the vanilla extract.
5. Gently add half of the flour mixture to the egg mixture and use a large metal spoon to fold gently until almost but not completely blended, then fold in the other half.
6. Spoon the batter into the (ungreased) cake tin, then pass a palette knife through the mixture to release any large air pockets. Bake in the centre of the oven for 35-40 minutes until a skewer inserted into the centre comes out clean.
7. As soon as the cake comes out of the oven, invert it onto a wire rack. Leave to cool in the tin for 30 minutes, then run a sharp knife around the edges to release it. Remove from the tin to cool completely.
8. Once the ganache has cooled completely, add a few cubes of the butter and use an electric whisk to beat on a low-medium speed until combined. Continue whisking while gradually adding the rest of the butter. Increase the speed and whip until light and fluffy. Spread the chocolate ganache frosting onto the cake and decorate with grated white chocolate.