Once the sponge cake has cooled completely, slice it into three layers keeping the top layer a bit thinner than the others. Start to assemble the prinsesstårta by spreading a thin layer of jam onto the bottom layer of the sponge cake. Add a generous layer of vanilla custard by either piping or spreading it over the jam. Add the next layer of sponge cake and another generous layer of vanilla custard. Whip the 2 cups (480ml) of heavy cream until stiff and pile a generous, fluffy mound onto the last custard layer. Add the final layer of sponge cake and gently form it into a dome shape over the cream being careful not to squish too much cream out of the sides. Spread a thin layer of cream over the top to even out and seal the surface. Gently lay the thin marzipan sheet over the top of the cake and use your hands to help define the shape. Trim the excess marzipan from the bottom of the cake by using a pizza or pastry wheel or a very sharp knife. Dust the top with sifted powdered sugar (you can use a tea strainer) and garnish with a pink marzipan rose with leaves. The cake needs to be refrigerated until serving time and any leftovers need to be refrigerated as well. Ideally the cake should be eaten the day it is made.