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Heat oven to 180C/160C fan/gas 4. Grease and line the base of a 20cm springform cake tin with baking parchment. Beat the butter and caster sugar together until pale, light and fluffy – this is easiest using a freestanding machine or an electric hand-held mixer. Gradually add the egg, a little at a time, beating well between each addition.
Sift both flours, the bicarbonate of soda and the cocoa into a bowl. Add half the sifted dry ingredients to the egg mixture and fold in using a large metal spoon. In another bowl, mix the soured cream and cream cheese together until smooth. Add half to the cake batter and fold in. Repeat this process with the remaining flour and soured cream, and mix until smooth. Pour into the cake tin. Bake on the middle shelf for 1 hr or until a skewer inserted into the middle of the cake comes out clean. Leave the cake to cool in the tin for 10 mins before turning out onto a wire rack and removing the baking parchment.
Meanwhile, make the icing. Using an electric mixer, beat the butter until pale and softly whipped. Add the icing sugar and whisk until smooth, then add the cream cheese and whisk again. Sieve the juice from the passion fruits, reserving the pulp and seeds. Add the juice to the icing.
To assemble, split the cake into 3 even layers and place the bottom layer on a cake stand or plate. Spread one-third of the icing over the bottom layer, swirling out to the edges. Repeat with the remaining 2 layers of cake and icing, finishing with a layer of icing. Top with the passion fruit pulp and seeds. Serve straight away. Will keep for 3 days, chilled – but bring back to room temperature before serving.