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Mix the sugars and almonds in a large bowl, then rub in the vanilla seeds until even. Make a well in the middle, then tip in the eggs and citrus juice. Cut the wet ingredients into the dry with a cutlery knife. Dust the surface with icing sugar, then knead the marzipan briefly with your hands to a smooth dough. Don’t overdo it as the paste can get greasy. Add a bit more icing sugar if it seems too wet. Shape into a ball, then wrap and keep in a cool place until ready to cover the cake. Can be made up to 2 days in advance.
Lift the cake onto a cake board or plate, then use a pastry brush to cover evenly with a thin layer of jam. Dust the work surface with more icing sugar, then roll the marzipan into a circle about 40cm across, dusting underneath the marzipan with more icing sugar and turning it a quarter turn after every few pushes of the rolling pin.
Flip the top of the circle back over your rolling pin so you can see the underside of the marzipan, then lift the pin up and lift the marzipan over the cake. Stop once you can see that the edge of the marzipan nearest you is about level with the bottom of the cake. Flop the front of the marzipan down. Smooth the paste over the cake using the palms of your hands, then trim with a sharp knife. If any cracks appear, simply pinch the paste back together and smooth. Leave to dry for at least 24 hrs, or up to 3 days, before covering with icing.
How long does homemade marzipan keep? Although homemade marzipan contains raw eggs, the amount of sugar, and lack of moisture, prevents bacteria growing when left at room temperature, so your cake should last for 1-2 months iced. The most important thing is that you don't scrimp on drying time – once you’ve covered your cake in marzipan it should be left to completely dry out, before you then cover it with icing. If you want to make marzipan before you are ready to cover your cake, then wrap it well in cling film and keep in a fridge for up to a week – as wrapped in a ball it will still contain a little moisture. As it contains raw eggs it shouldn’t be given to anyone in an at-risk group, including pregnant women, the elderly, the unwell and the very young. You can make a cooked marzipan, which carries no risks. It is a little softer, and harder to work with, but a good substitute if you are at all worried.