For the nougatine base, line a baking tray with a silicone mat. Lightly grease a 25cm/10in flan tin.
Sprinkle the flaked almonds onto a baking tray and bake in the oven for 7-8 minutes.
Heat the fondant icing and glucose in a large saucepan over a medium heat and boil for 10-15 minutes, or until a pale caramel colour. Stir in the toasted almonds and butter and pour onto the lined baking tray. Place another silicone mat on top, then roll out the nougatine to a thickness of 5mm/¼in then remove the top mat and carefully lift the nougatine into the greased flan tin, pressing it into the sides. Trim off the excess with scissors then set aside to cool.
For the choux buns, increase the oven to 200C/400F/Gas 6. Grease several large baking trays.
Put the water, milk, butter, condensed milk and salt into a large saucepan and bring to a rolling boil. Tip in the flour and beat with a wooden spoon until smooth.
Continue cooking, stirring constantly over a moderate heat for about 3-4 minutes or until there is a thin film of dried dough on the sides and bottom of the pan.
Transfer the dough to the bowl of a food mixer and beat with the paddle attachment on a medium speed until steam is no longer rising from the bowl. Reduce the speed to low and beat in the eggs, one at a time; continue beating until the dough is smooth and glossy.
Spoon the dough into a large piping bag fitted with a plain nozzle and pipe 5cm/2in balls onto the baking trays, leaving room for them to spread a little as they cook.
Make the egg wash by beating the egg yolk with the water. Brush the balls with the egg wash and bake in the oven for 10 minutes, then reduce the oven to 180C/350F/Gas 4 and bake for 15 minutes more or until the buns are well risen and golden-brown. Remove from the oven and set aside to cool.
To make the pastry cream, place the vanilla seeds and milk in a large saucepan, bring to the boil then remove the pan from the heat.
Place the sugar and egg yolks in the bowl of a mixer fitted with a whisk attachment and whisk on a high speed for about three minutes, or until a thick ribbon drips off the whisk, then reduce the speed to medium and beat in the cornflour.
Whisk in the hot milk, then pour the mixture back into the saucepan and heat gently until boiling, stirring all the time, then let it bubble for one minute or until thickened and pour into a clean bowl. Cover the surface with cling film to prevent a skin forming and set the bowl aside to cool.
For the praline paste, toast the almonds and hazelnuts in a dry pan over a gentle heat for about 10 minutes, taking care not to burn them, then place the toasted nuts on a baking tray lined with a sheet of lightly greased greaseproof paper.
Heat the water and sugar in a saucepan over a high heat, swirling from time to time, until the mixture is a pale caramel colour. Pour the caramel over the nuts and set aside to cool completely.
Break the praline into pieces and blend to a paste in a food processor. Mix the praline paste into the pastry cream and spoon into a piping bag fitted with a small nozzle. Cut a small hole in the bottom of each choux bun and pipe in a little of the praline mixture.
To construct the croquembouche, remove the hardened nougatine from the tin and turn upside down to form the base for the croquembouche.
Make the caramel by dissolving the sugar in a pan with the cardamom and water and heating to a pale caramel. Dip the filled choux buns into the caramel and stack the choux buns on the nougatine base in a pyramid shape.
If the caramel becomes too stiff to work, return it to the heat until it loosens. Dip some of the buns in the sugar crystals to decorate as you go.
For the spun sugar decoration, cut the ends off the bottom of a wire whisk with pliers to make a brush.
Return the remaining caramel to the heat then dip the brush in the caramel and shake vigorously back and forth over a silicone mat. The caramel will fall in thin strands and set. Gather the strands up and arrange on top of the croquembouche.