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Heat the oven to 180C/160C fan/gas 4. Butter two round 18cm cake tins and line with baking parchment. Beat the butter and xylitol together in a large bowl using an electric whisk until light and creamy. Add the egg, a little at a time, beating well after each addition. Fold in the flour and a pinch of salt. Divide the batter evenly between the two tins and smooth the surfaces using a spatula. Bake for 15-20 mins until golden and a skewer inserted into the middles comes out clean. Leave to cool completely in the tins, set on a wire rack.
If using defrosted raspberries, first tip them into a sieve over the sink and use the back of a spoon to extract as much liquid as possible (frozen berries contain a lot of water, and you don't want the filling to be too wet). Put the raspberries in a small bowl and mash with the xylitol. Set aside.
Lightly whip the double cream, if using, until just holding its shape. Remove the parchment from one of the sponges and put on a cake stand. Spoon over the crushed raspberries in an even layer. Sandwich with the second cake, then spread over the whipped cream in thick swirls. Chill until ready to serve.