Start making this cake at least 24 hours before you plan to serve it. This gives you plenty of time to make all of the baked layers in advance, to chill the ice-cream mixtures before churning and for the ice cream layers to freeze.
Heat the oven to 170°C/150°C fan/Gas 3.
Make the Christmas cake. Beat together the butter and sugar in a stand mixer fitted with the beater, on medium speed, for 3–4 minutes until light and fluffy. Add the eggs, flour, baking powder, xanthan gum and spices and beat until smooth and thoroughly combined. Add the remaining ingredients and mix to combine.
Divide the mixture equally between the prepared cake tins (800g per tin) and spread level. Bake for 25 minutes, or until a skewer inserted into the centres comes out clean. Turn out the cakes onto wire racks and leave to cool completely. Clean the cake tins to use later.
Once cooled, wrap one cake layer in cling film and place it in the freezer and cover the second cake layer and set it aside at room temperature, while you prepare the remaining layers.
Make the Christmas pudding ice-cream. In a large bowl whisk the egg yolks, dark muscovado sugar, salt and cornflour until smooth.
In a large pan, heat the milk, cream and vanilla until just boiling. Pour the milk mixture over the egg-yolk mixture, whisking continuously. Return this to the pan and cook over a low heat until just boiling, stirring until the custard has thickened enough to coat the back of a spoon and the bubbles have subsided. Strain into a bowl, cool, then cover chill for at least 2 hours.
Make the brandy ice cream. Pour 500ml of the milk into a pan and bring to the boil. Meanwhile, in a bowl whisk the remaining 100ml milk with the sugar, milk powder, cornflour and orange zest.
Pour the hot milk into the bowl, whisking continuously until smooth. Return the mixture to the pan and slowly bring it back to the boil, whisking continuously until just boiling and thickened. Pour the mixture into a clean bowl, add the brandy and vanilla, mix to combine and leave to cool. Cover and chill for at least 2 hours.
While the ice-cream mixtures are chilling, prepare the brandy snaps. Heat the oven to 190°C/170°C fan/Gas 5. Line each 23cm cake tin with a 25cm disc of baking paper, so that the paper comes a little way up the sides of the tins.
In a small pan over a low heat, melt the butter with the demerara sugar and golden syrup and stir to dissolve. Remove from the heat, add the flour and cinnamon and mix until smooth. Divide the mixture between the lined cake tins (100g per tin), spreading it level with the back of a spoon. Bake for 8–9 minutes, until golden and lacy. Leave to cool and harden and then remove from the tins.
Melt 150g of the chocolate either in the microwave or in a bowl set over a pan of simmering water. Stir until smooth, then brush a thin layer of chocolate on the top of each brandy snap. Place in the fridge for 10 minutes, until the chocolate has set, then flip the brandy snaps over and brush chocolate onto the other sides.
Line the base and sides of one 23cm cake tin and the springform tin each with a double layer of cling film. Unwrap the frozen cake layer and place it in the bottom of the springform tin. Place one chocolate-coated brandy snap in the other lined tin. Return both tins to the freezer.
Once it’s completely cold, churn the Christmas-pudding ice-cream mixture in the ice-cream machine according to the manufacturer’s instructions. Fill 12 of the silicone hemisphere moulds with ice cream and place them on a tray in the freezer.
Working quickly, crumble up 300g of the remaining Christmas cake into the remaining ice-cream and stir to combine. Spoon half of this mixture over the frozen cake layer and top with the second brandy snap. Spoon the remaining ice cream over the frozen brandy snap and return both tins to the freezer.
Meanwhile, churn the brandy ice cream in the ice-cream machine and make the marzipan. Combine the ground almonds and both sugars in a mixing bowl. Gradually add the egg and mix to combine until the mixture is no longer sticky, but smooth and pliable (you may not need all the egg). Lightly dust the work surface with icing sugar and knead the marzipan until smooth. Cover and set aside until needed.
Fill the remaining 12 hemisphere moulds with the churned brandy ice cream and return the moulds to the freezer. Spoon the remaining ice cream into the deep cake tin on top of the cake, ice cream and brandy-snap layer and return to the freezer for at least 6 hours, but preferably overnight.
Cut off one quarter of the marzipan, cover and set aside. Lightly dust the work surface with icing sugar and cornflour and roll out the remaining marzipan. Using a cake tin as a guide cut out a 23cm disc.
Lay the marzipan disc on top of the ice-cream layer in the tin with just brandy snap and ice cream and freeze again while you prepare the marzipan holly leaves.
Divide the reserved marzipan in half and use the food colourings to tint one half red and the other green. Using a holly-leaf cutter, stamp out leaves from the green marzipan and roll the red marzipan into small balls to resemble holly berries.
Turn the marzipan-topped ice cream layer out of the tin, peel off the cling film and place it on top of the ice-cream layer cake in the deep tin, inverting it again so that the marzipan is on the top of the stack. Cover and freeze again for at least 2 hours.
Meanwhile, turn the frozen ice-cream hemispheres out of the moulds and sandwich them together to make 12 balls. Return to the freezer on the lined baking tray.
Melt 250g of the remaining dark chocolate (either in a microwave or in a bowl set over a pan of simmering water), pour it into a mug and leave it to cool slightly. Working quickly, push a skewer or cocktail stick into each ice cream sphere and dip it into the melted chocolate to coat, allowing any excess to drip off. Remove the cocktail sticks and return the coated ice-cream spheres to the freezer.
Melt the white chocolate (either in a microwave or in a bowl set over a pan of simmering water), leave it to cool slightly and spoon it into the piping bag fitted with the 5mm nozzle. Drizzle the top of each chocolate-coated ice cream sphere with white chocolate and top with marzipan holly leaves and berries. Return to the freezer until needed.
To make the crystallized pistachios, dissolve the sugar in 1 teaspoon of water in a small pan. Bring to the boil, add the pistachios and stir to coat. Turn out the crystallized pistachios onto a lined tray and leave to harden.
To decorate the cake, first make the Italian meringue. Tip the caster sugar into a small pan, add 4 tablespoons of water and place the pan over a medium heat to dissolve the sugar. Bring to the boil and continue to cook until the syrup reaches 110°C on the sugar thermometer. Start whisking the egg whites in a stand mixer fitted with the whisk, until they reach soft peaks.
As soon as the syrup reaches 120°C, remove it from the heat and pour it in a steady steam onto the egg whites, whisking continuously. Continue to whisk until the meringue is stiff, glossy white, and cold.
Place a baking sheet on top of the ice cream cake, turn it over and remove the cake from the tin. Peel off the cling film, place a serving plate on top of the cake and flip it back over, so that the cake layer is now on the bottom and the marzipan on top again.
Cover the top and sides of the cake in meringue, making swoopy, snowy peaks. Freeze the cake again for 30 minutes.
Just before serving, remove the cake from the freezer and decorate the top with ice-cream spheres, crystallized pistachios, orange peel, cinnamon sticks, star anise, rosemary sprigs, redcurrant sprigs and dried cranberries, and serve immediately.