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Cover the dates with boiling water and set aside to soften for 30 mins.
Heat oven to 160C/140C fan/gas 3 and line a 20 x 30cm baking tin with baking parchment. Drain the dates, reserving the liquid, then whizz the dates to a purée with 100ml of the soaking liquid.
Beat the egg whites in a large bowl with an electric whisk until stiff peaks hold on the end of your whisk. Add half the sugar and beat until thick and glossy.
In another bowl, beat the yolks and remaining sugar until pale. Whisk in the mashed date mixture, then fold into the egg white mixture until well combined. Mix the flour, baking powder and cocoa with a pinch of salt. Sprinkle over the wet mixture and very gently fold in until well combined. Gently scrape into the tin and spread to the edges. Bake for 35-40 mins until a skewer poked in comes out clean.
Meanwhile, mix the custard powder, sugar and cocoa together in a saucepan. Stir in dribbles of milk until you get a smooth paste. Gradually add more milk until it’s all incorporated with no lumps. Put over a medium heat and warm, stirring constantly, until thick and just bubbling.
Cut the warm cake into slabs and pour over the hot chocolate custard to serve.