Heat the oven to 180C/160C fan/gas 4. Oil and line two round 20cm springform cake tins with baking parchment. Whisk the oil, sugar, eggs and milk in a bowl until smooth. Sieve the flour, cocoa and bicarb into another large bowl, then gradually mix in the wet ingredients.
Divide the mixture between the tins and bake for 35-40 mins until the cakes are risen and spring back when pressed. Leave to cool in the tins for 10 mins, then transfer to a wire rack to cool completely.
For the raspberry layer, stir the jam and the defrosted raspberries together. Once the cakes are cool, whip the cream with the sugar to soft peaks, then gently fold half the raspberry mixture through the cream to create a ripple effect.
Spoon most of the reserved raspberry mixture over one of the cakes, then dollop on half of the cream. Smooth over with a palette knife, then place the other sponge on top. Swirl over the remaining cream and swirl the last of the raspberry mixture through it. Will keep in the fridge for two days.