This towering centrepiece of choux buns is a showstopper to serve at a special celebration. You can prepare the buns, filling and nougatine in advance, but make sure you do the the final assembly as close as possible to serving.
Make the craquelin. In a bowl, cream the softened butter with the demerara sugar until thoroughly combined. Add the flour and mix until smooth. Flatten the craquelin into a disc and roll it between two sheets of baking paper until 1–2mm thick. Chill on a baking sheet for 30 minutes.
Make the choux buns. Pour the milk, 125ml of water, and the butter, sugar and salt into a medium pan over a medium heat. Stir to melt the butter and then quickly bring the mixture to a rolling boil. Immediately slide the pan off the heat, quickly add the sifted flour and beat vigorously until the batter is smooth. Return the pan to a medium heat, beating continuously for 20 seconds, until the mixture is glossy and cleanly leaves the sides of the pan.
Tip the mixture into a bowl and leave to cool for 5–10 minutes. Gradually add the eggs, beating well after each addition, until the batter is silky smooth and reluctantly drops off the spoon. You may need only 5½ eggs. Scoop the dough into a piping bag fitted with a plain 1.5cm nozzle and pipe 30–35 small mounds, each the size of a strawberry, over each lined baking sheet. Leave a little space between each mound to allow for expansion during cooking.
Remove the craquelin from the fridge. Using the cutter, stamp out discs, re-rolling the trimmings to cut out 30–35 discs (one for each bun). Place a craquelin disc on top of each choux bun on the baking sheets.
Cook the choux buns for 25–30 minutes, until puffed, crisp and golden brown. Using the wooden skewer, make a hole in the underside of each bun to allow the steam to escape and leave the buns on the baking sheets to cool completely.
Make the crème diplomat. Heat the milk in a medium pan until just boiling. In a mixing bowl whisk together the sugar, egg yolks and cornflour until smooth and creamy. Whisking continuously, pour half of the hot milk into the bowl and continue to whisk until smooth. Pour the contents of the bowl back into the pan and whisk over a low heat until the mixture starts to bubble and thicken. Continue to cook for 2–3 minutes to cook out the cornflour – you should not be able to taste any flouriness in the mixture.
Remove the mixture from the heat, add the butter and sea salt and mix until combined and glossy. Strain the crème into a clean bowl and cover the surface with cling film to prevent a skin forming. Leave until cold, then chill for about 4 hours.
Tip the blackberries into a pan, add the sugar and lemon juice and cook over a low–medium heat, until very soft. Pass the fruit pulp through a fine sieve, pushing through all of the juice and pulp. Discard the seeds. Return the resulting purée to the pan and cook over a low heat, stirring frequently until reduced by half. Cool and chill until needed.
Make the nougatine. Tip the caster sugar and glucose into a heavy-based medium pan and add 4 tablespoons of water. Place the mixture over a low heat and leave the sugar to dissolve – you can gently swirl the pan from time to time, but don’t stir. Bring the syrup to the boil and continue to cook, without stirring, until the syrup becomes a deep, amber-coloured caramel. Add the flaked almonds and butter and stir to thoroughly combine. Working quickly, scoop the nougatine into the lined tins and using a lightly buttered or oiled spoon spread it into an even layer in each. Leave to cool and harden.
Divide the choux buns into thirds – one third will be topped with white pearl sugar and another with raspberry pearl sugar and the third will remain uncoated. Put the white and raspberry pearl sugars on separate plates, ready for dipping.
Tip 200g of the caster sugar for the decoration into a small pan and place over a low heat to melt the sugar, swirling but not stirring. Continue cooking the sugar until it turns to a toffee-coloured caramel. Slide the pan off the heat and plunge the base into a bowl of cold water to stop the caramel cooking any further. Working quickly and carefully and one at a time, dip the top of one third of the choux buns into the caramel and then press them into the white pearl sugar. Leave to dry, sugar-side uppermost on a lined baking sheet. You may need to remelt the caramel over a low heat if it cools and hardens. Dip and coat the second third of buns in raspberry pearl sugar.
To fill the buns, whip the cream until it holds firm peaks and fold it into the chilled custard. Divide the mixture in half and add the blackberry purée to one half and the lemon juice and grated lemon zest to the other. Spoon the fillings into two piping bags fitted with 5mm nozzles. Fill half the buns with lemon crème and half with blackberry, each time piping into the hole on the underside of each bun.
Assemble the tower (do this just before serving). Caramelise the remaining 200g sugar using the method in Step 11. Spoon 1 teaspoon of caramel onto the serving plate and place the 23cm disc of nougatine on top, using the caramel as glue.
One at a time, dip one edge of the choux buns into the caramel and place it caramel-side downwards in a neat circle around the edge of the nougatine and with the pearl sugar or craquelin top facing outwards. Fill the middle of the disc with a layer of buns and then build a second layer.
Place the 20cm nougatine disc on top and repeat, positioning more choux buns – remelting the caramel as needed. Continue to build the tower, until you have used up all the discs (getting narrower each time) and all the choux buns, finishing with a conical top of buns. Serve soon after assembly.