Remove cooled cake from the freezer. Using a bread knife, carefully split layer into two layers, cutting all of the way through. Remove top layer, carefully, to wax paper with the cut side up. Carefully, loosen the bottom of the springform pan from the bottom layer by sliding knife or spatula between parchment paper and pan bottom. Then slide cake onto a cake plate with the cut side up.
Using a pastry brush or a spoon, put limoncello soaking liquid on each layer. (This is why I use the springform pan and cut the layers instead of using two or three round cake pans. The cut layers really allow the liquid to absorb into the cake.)
Using half of the mascarpone mousse on the layer of cake on the cake plate, smooth it out, and then place it into the freezer until well set. If you rush this, the mousse will squeeze out when the other layer is placed upon it. Return other layers to the freezer also.
Carefully, place the top layer onto the frosted bottom layer.
Spread mousse along the top of the cake. You can frost the sides of the cake if you want, but I do not always do that.
Grate white chocolate on top of the mascarpone mousse. Chill cake for several hours. It takes a while for the mousse to firm up and it's even better the day after it's assembled.
GARNISHES: When slicing, you may garnish with fresh mint and a lemon slice. Or, it looks really great garnished with a raspberry and some raspberry sauce.