Chilling the flour (as well as the butter) before using it helps to ensure that you end up with really flaky, tender layers of puff pastry for your Saint Honoré gâteau – making this recipe a fitting tribute to the patron saint of baking, after whom it takes its name.
For the puff pastry, put both flours as well as the salt, egg and water into a bowl and use your fingers to gently mix to an even dough. Transfer the dough to a lightly floured surface and knead for 5–10 minutes, until smooth. Shape the dough into a ball, wrap in cling film and chill for 20 minutes.
Using a rolling pin, flatten the butter between 2 sheets of cling film to a rectangle measuring about 18 x 13cm. Place in the fridge to chill with the dough.
When ready, roll out the chilled dough to a rectangle measuring about 30 x 15cm. With a short end of the dough closest to you, position the butter so that it covers the bottom two thirds of the dough. Make sure that it is positioned neatly and covers almost to the edges.
Lift the exposed dough at the top and fold it down over half of the butter, then fold the butter-covered bottom half of the dough up over the top. You will now have a sandwich of two layers of butter and three of dough. This is called a single turn. Pinch the edges together to seal. Wrap in cling film and chill in the fridge for 30 minutes.
Unwrap the dough and, with a short end closest to you, roll out on a lightly floured surface, to a rectangle as before, keeping the edges as even as possible. Fold the top quarter down and the bottom quarter up so they meet neatly in the centre. Then fold the dough in half along the centre line where the edges meet. This is called a book turn. Wrap in cling film and chill for 30 minutes.
Unwrap the dough and, with a short end closest to you, roll out on a lightly floured surface to a rectangle as before and fold as in the single turn in step 4. Wrap in cling film and chill for 30 minutes.
Heat the oven to 220°C/200°C fan/Gas 7. On a lightly floured surface, roll out the pastry to a 28cm square, then place it on a lined baking tray and freeze for 10 minutes.
Place a sheet of baking paper on top of the rolled pastry and place a large (unlined) baking sheet on top of that. Bake the pastry sheet like this for 15–20 minutes, until golden-brown and crisp when you peep beneath the tray. Set aside to cool.
For the crème Chiboust, soak the gelatine leaves in a small bowl of cold water until soft. Pour the milk into a pan and scrape in the vanilla seeds (you can put the remaining pod in a jar of caster sugar to make vanilla sugar, if you like). Bring to the boil, then remove from the heat and leave to cool for 30 seconds. Meanwhile, whisk the egg yolks and 50g of the caster sugar in a stand mixer fitted with the whisk, until pale, then whisk in the cornflour. With the whisk on a slow speed, pour in the hot vanilla milk, whisking continuously until combined, then pour the mixture back into the milk pan.
Bring the mixture back to the boil, whisking continuously over a medium heat for 2–3 minutes. Squeeze the gelatine to remove any excess water, then add to the pan and whisk until smooth. Remove from the heat and pour into a bowl. Cover the surface with cling film to prevent a skin from forming and leave to cool to room temperature.
Once the crème is cool, tip the egg whites into the bowl of a stand mixer fitted with a whisk and whisk to stiff peaks. Add the remaining 50g of sugar, 1 tablespoon at a time, whisking continuously until you have a stiff and shiny meringue. Add a large spoonful of the meringue to the cooled crème and whisk until mixed. Fold in the remaining meringue until incorporated. Spoon two thirds of the mixture into the piping bag fitted with the jam nozzle and the remaining one third into a piping bag fitted with a Saint Honoré nozzle. Chill until needed.
For the choux pastry, reduce the oven to 190°C/170°C fan/Gas 5. Using the 2.5cm cutter or the large end of a piping nozzle, draw 14 well-spaced circles on the sheets of baking paper. Turn the sheets over, so the ink or pencil is underneath.
Sift the flour into a bowl. Put the butter and 100ml of water into a medium pan over a low heat. Allow the butter to melt completely, but don’t let the liquid boil.
Once the butter has melted, increase the heat, bring to the boil and immediately remove from the heat. Quickly add the sifted flour in one go. Using a wooden spoon, beat the mixture to a smooth, glossy dough. Return the pan to the heat and beat for 1 minute, until the dough is very thick and forms a ball that leaves the sides of the pan clean.
Tip the dough into a stand mixer fitted with the beater and leave until barely warm, then gradually add the eggs, beating well between each addition, to a dropping consistency.
Spoon the dough into the piping bag fitted with the 1.5cm plain nozzle and pipe 28 small circles on the two lined baking sheets, using the drawn circles as a guide. Bake for 15–20 minutes, until puffed, crisp and golden. Turn off the oven and leave the choux buns in the residual heat for 10 minutes to dry out. Remove from the oven and transfer to a wire rack to cool.
Once cooled, pierce a hole in the bottom of each choux bun with the tip of a knife and fill with crème Chiboust, using the portion in the piping bag with the jam nozzle. Set aside.
For the caramel, melt the sugar and 50ml of water together in a heavy-based pan over a medium heat. Add the glucose, then cook until the caramel is a deep amber colour. Remove from the heat and plunge the base of the pan into cold water to prevent the caramel burning. Take care: the caramel will be very hot.
Using tongs, carefully dip the top half of each filled choux bun into the caramel, then transfer, caramel side upwards, onto the remaining baking tray lined with baking paper. Leave to set. If the caramel in the pan starts to harden, return it to the heat to melt.
For the crème Chantilly, whisk the double cream, whipping cream, icing sugar and vanilla extract until just thick enough to pipe. Spoon into the remaining piping bag fitted with a St Honoré nozzle.
To assemble, trim the sheet of puff pastry and cut into 2 rectangles, each measuring about 24 x 12cm. Place one of the pastry sheets on a serving plate. Pipe a wide ribbon of crème Chiboust (from the piping bag fitted with the Saint Honoré nozzle) down the middle of the pastry, leaving enough space for a row of choux balls down each side. Pipe a dot of crème Chiboust on the base of each filled choux bun and stick them either side of the piped ribbon, 7 on each side.
Place the second sheet of pastry on top of the choux buns and pipe the remaining crème Chiboust down the middle, leaving just enough to stick the last 14 choux buns down the sides (again, 7 on each side). Pipe the crème Chantilly in the middle to decorate.