Preheat the oven to gas 4, 180°C, fan 160°C. Lightly grease 2 x 20cm (8in) loose bottomed tins with butter and line each base with nonstick baking paper.
Beat the butter and sugar with an electric whisk for about 5 minutes, until light and fluffy. Gradually add the eggs, one at a time and beat well between each addition. Fold in the flour and baking powder, using a large metal spoon, then add the orange zest and juice. Divide the mixture between the tins and level the surface. Bake for 20 minutes, until the cake is spingy to touch
Remove the cakes from the oven, leave to cool for 5 minutes in the tin and then turn out onto a wire rack and leave to cool completely.
To make the frosting, beat the mascarpone with the icing sugar and Grand Marnier or orange juice using an electric whisk, until smooth and creamy. Set aside.
Cut each cake in half horizontally to create 4 sponge layers. Place one layer on a serving plate, then spread with half the blackberry jam, then scatter over 200g (7oz) of the blackberries, squashing them down slightly with a fork. Top with another layer of cake and spread with the raspberry jam, followed by 200g (7oz) raspberries, squashed down slightly with a fork. Add the third layer of cake and spread with the rest of the blackberry jam and another 200g (7oz) blackberries. Position the remaining layer of cake on top to finish off
Using a palette knife or spatula, spread the mascarpone frosting evenly over the the cake. Decorate with some extra fresh raspberries and blackberries and white chocolate curls.