For the cake, butter and line three 18cm/7in cake tins with greaseproof paper. Preheat the oven to 180C/350F/Gas 4.
Beat the sugar and butter in a bowl until light and fluffy, then mix in the eggs, one at a time.
Add the flour and the rest of the ingredients, mixing well with a large metal spoon.
Divide the mixture among the three tins and bake for 30-40 minutes, or until the cakes are golden-brown and firm to the touch. Remove from the oven, cool for 10 minutes in the tin and then turn out and cool completely.
For the frosting, blend the blackberries in a food processor or blender until smooth. Stir in blackberry compôte, to taste.
Beat the mascarpone in a bowl until smooth, add the cream and the icing sugar.
Mix in the blackberries with a spoon, taste and add more sugar to taste if necessary.
For the decoration, whisk the double cream until soft peaks form when the whisk is removed from the bowl. Stir in the icing sugar. Transfer the cream into a piping bag.
Assemble the cake by smoothing a layer of the frosting on one cake disc, top with some of the soft fruit, top with another disc of cake, more fruit and then the final disc.
Spread the remaining frosting all around the cake and arrange more fruit on top. Top with milk chocolate and white chocolate flakes. Pipe the cream around the top of the cake.