On a baking sheet lined with parchment paper, bake the chopped walnuts in the oven at 350°F until they turn golden brown. Remove them from the oven and allow them to cool at room temperature.
Using a stand mixer or hand mixer, beat the cream cheese with the condensed milk until smooth. Next, add in the heavy whipping cream and beat on high until the mixture expands 3 times in volume. Make sure to not over beat the mixture, as it will turn runny.
Slice each sponge cake into two even layers.
Place one cake layer on a serving plate. Spread a bit of the white cream on top, followed by half of the dried plums.
Place the second layer on top and spread on a layer of cream, followed by a handful of walnuts. Next, pop on the third layer and spread it with the white cream, followed by the other half of the dried plums. Cover the cake with the last layer of sponge cake.
Frost the entire outside of the cake with the remaining cream and cover the sides with walnuts. Additionally, sprinkle some walnuts alongside the rim of the top of the cake.
Allow the cake to refrigerate overnight before serving. Slice and enjoy!