Heat oven to 180C/160C fan/gas 4. Grease and line the bases of 2 x 20cm sandwich tins with baking parchment. Using an electric hand whisk, beat together the caster sugar and butter until light and fluffy. Add the eggs one at a time, beating well between each addition.
Sieve the flour and cocoa powder into a bowl. Add half to the egg mixture and fold in carefully. Mix together the buttermilk, vinegar, cream cheese, raspberry puree and food gel. Fold half the buttermilk mixture into the cake batter and repeat the process with both the flour mixture and buttermilk mixture.
Spoon the combined mixtures into your prepared tins and cook for about 25-30 mins, until an inserted skewer comes out clean. Leave to cool for 10 mins before turning out onto a wire rack.
For the frosting, mix together the icing sugar and butter until soft and creamy, then stir in the cream cheese. Sandwich the cooled cakes together with half of the frosting, adding 150g fresh raspberries if you like, then spread the remaining frosting over the top. Will keep for up to 3 days in a cool place.