Slice panettone horizontally into four rings, then place the bottom piece on a serving plate. Whisk mascarpone, cream and icing sugar until soft peaks begin to form. Add half the raspberries and beat until quite firm. Mix the remaining berries and cherries together. Spread a third of the cream on the base and scatter a third of the berries and cherries on top. Lay another slice of panettone on top and repeat until you have reassembled the cake. Cover and chill for 2 hours. To serve, dust panettone with extra icing sugar and then cut it into wedges.