Place the heavy cream in a large bowl. Add 2 tablespoons sugar and whisk together until firm. In another bowl add the blueberries, raspberries, blackberries, strawberries, remaining sugar, and grappa. Allow the mixture to rest for 5 minutes and allow the sugar to marinate with the grappa. Cut the Panettone cake into approximately 1-inch to 1 1/2-inch thick slices. Reconstruct the Panettone by re-layering the slices of cake with the whipping cream and mixed fruit and juices equally in between all the layered pieces. Repeat until all the layers and cake have been reconstructed. Add any remaining juices and an optional splash of grappa on top.