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Heat oven to 160C/140C fan/gas 3. Grease and line the base and sides of a 20cm deep cake tin with baking parchment. Put the butter, sugar, flour, eggs, soured cream, vanilla and 1⁄4 tsp salt into a large mixing bowl and blend together using an electric hand whisk. Once smooth, scrape into the cake tin and bake for 50-55 mins until golden and risen, and a skewer inserted in the centre comes out clean. Leave to cool for 10 mins in the tin, then transfer to a wire rack to cool completely.
Clean the cake tin, and line with a double layer of cling film. Tip half the raspberries, half the redcurrants and 2 tbsp of the icing sugar into a bowl and lightly crush with a fork, set aside. Pour the double cream, icing sugar and mascarpone into another bowl and whisk with an electric hand whisk until softly whipped and just holding its shape. Fold through the sour cream, peaches, and whole and crushed berries.
Split the cake into 3 even layers. Flip the top layer into the cake tin, so the top now becomes the base. Scrape half the cream and fruit mixture into the tin, spread to the edges and level the top. Flip the middle layer of cake into the tin, top with the remaining cream and fruit mixture, then flip on the final layer of sponge. Give the sponge a gentle press to expel any gaps between the layers, then overwrap and place in the freezer. You can also freeze some extra raspberries and redcurrants to serve alongside. Freeze the cake for at least 4 hrs.
Remove the cake from the freezer 20 mins before serving. Tip it out of the tin onto a cutting board and slice. Any leftovers can be put back in the freezer and eaten over the next 2 weeks. Serve with extra berries and a berry coulis (recipe in 'goes well with').