Once the chocolate sponge cake is fully chilled, use a long serrated knife to carefully slice it horizontally into three even layers .
Place the bottom layer on your serving plate or cake stand. Spread an even layer of raspberry jam over it .
Pipe or spread the chilled vanilla custard on top of the jam, leaving a small border around the edge .
Place the second cake layer on top of the custard.
Now, create the dome. Mound the chocolate whipped cream onto the center of the second cake layer, using a spatula to shape it into an even dome .
Place the third and final cake layer on top of the cream dome, gently pressing it down to secure it. You can trim this layer to be slightly smaller to help with the dome shape.
On a surface dusted with powdered sugar, roll out the green marzipan into a large circle, about ⅛-inch (3-4 mm) thick. It needs to be large enough to cover the entire cake .
Carefully lift the marzipan and drape it over the cake, smoothing it down gently over the dome and around the sides. Trim any excess marzipan from the bottom.
For a classic finish, dust the top with a little powdered sugar. You can also craft a small pink marzipan rose for decoration if you feel adventurous .
Chill the finished cake for at least an hour before serving to allow the layers to set. Enjoy your masterpiece