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Preheat the oven to fan160C/ conventional 180C/gas 4 and butter and line the base of a loose-based 22cm round cake tin with non-stick baking paper or reusable Bake-o-glide.
Put the butter, sugar, eggs,flour, baking powder and vanilla in a bowl. Beat with a wooden spoon for 2-3 minutes, or with a hand electric beater for 1-2 minutes, until lighter in colour and well mixed. Beat in 4 tbsp soured cream, then stir in half the blueberries with a large spoon.
Tip the mixture into the tin and spread it level. Bake for 50 minutes until it is risen, feels firm to the touch and springs back when lightly pressed. Cool for 10 minutes, then take out of the tin and peel off the paper or lining. Leave to finish cooling on a wire rack.
To make the frosting, beat the soft cheese with the icing sugar and the remaining soured cream in a bowl until smooth and creamy. Spread over the top of the cooled cake (don't be impatient as the frosting will melt if the cake is too warm) and scatter with the remaining blueberries. The cake will keep in the fridge for a couple of days. Bring it to room temperature for about an hour before serving.