A genoise sponge base soaked in Grand Marnier and topped with oodles of custard and candied orange slices.
Using a mandolin or very sharp knife, slice the orange very thinly, about ¼cm thick. Tip the sugar and water into a large pan and bring to the boil, stirring until the sugar has dissolved. Add the orange slices and bring to the boil. Reduce the heat, cover and simmer over a very low heat for 2 hours. Remove from the heat and leave to cool. Drain the oranges, reserving the sugar syrup.
Mix 100ml of the reserved sugar syrup with the Grand Marnier and set aside. Reserve any remaining sugar syrup for glazing.
For the crème patissiere, measure the milk into a medium pan and add the vanilla paste. Bring to just below boiling point.
In a bowl, whisk the eggs, sugar and flour together. Pour the hot milk over the egg mixture, whisking continuously. Return the mixture to the pan, then cook over a low heat, stirring continuously, until thickened. Pour into a clean bowl and cover the surface with cling film. Leave to cool then chill in the fridge until set.
When set, whip the cream to soft peaks. Whisk the cooled crème patissiere until smooth, then fold in the cream.
For the genoise, pre-heat the oven to 180°C/160°C fan/Gas 4. Grease a 23cm springform tin with butter and line the base with baking parchment. Gently melt the butter in a small pan and set aside to cool slightly. Measure the eggs and sugar into a large bowl and whisk at high speed until the mixture is pale and mousse-like, and thick enough so that a trail is left when the whisk is lifted from the mixture.
Carefully fold half the flour into the egg mixture the gently pour half the cooled butter around the edge of the mixture and fold in. Repeat with the remaining flour and butter. Pour the mixture into the prepared tin. Bake in the pre-heated oven for 25–30 minutes or until well risen and the top of the cake springs back when lightly pressed with a finger. Leave to cool in the tin for a few minutes then turn out, peel off the parchment and finish cooling on a wire rack.
To assemble, line a 2-litre, 23cm diameter bowl or deep pie dish with a double layer of clingfilm. Slice the cooled genoise horizontally through the middle, so you have 2 sponges. Trim one of the sponges to an 18cm circle, using a plate or a cake tin as a guide.
Arrange the orange slices in the clingfilm-lined bowl. Place one slice in the centre and arrange 6 slices in a circle around the central slice. Cut 3 orange slices in half and use the 6 halves to fill the spaces left between the 6 round slices.
Spoon half of the crème patissiere into the bowl and gently smooth it over the oranges and up the sides to cover. Place the 18cm sponge on top and brush with half of the Grand Marnier syrup.
Finely chop the remaining orange slices and fold through the remaining crème patissiere. Spread the mixture over the soaked sponge. Top with the remaining sponge and brush with the remaining Grand Marnier syrup.
Cover with a sheet of baking parchment and place a 20cm plate or cake-tin base on top and press down gently. Chill for at least 6 hours, preferable overnight.
To serve, invert the bowl on a serving plate and turn out. Remove the clingfilm and brush the oranges with the reserved sugar syrup to glaze. The cake can be made up to 2 days ahead. Make up to step 12 and store in the fridge.