Begin by making the craquelin, mix the flour, sugar and butter together in a bowl, using your fingertips rub the ingredients together until they resemble breadcrumbs.
Then press the breadcrumbs together until they form a dough. Place the dough between two sheets of greaseproof paper and roll out the dough until it's 2mm thick. Place the dough on a baking tray, still covered in the greaseproof paper and put it in the freezer.
After about 30 minutes, remove the craquelin from the freezer and cut out lots of 3cm rings using a cookie cutter, you should fill the sheet with circles as you need a lot. Return the craquelin discs to the freezer until you are ready to use them.
Next make the choux pastry, preheat the oven to 180C (160C Fan) Gas 4 and line three baking trays with baking parchment.
Put the butter, salt, sugar and 120ml water in a medium pan over a medium-high heat. Once the butter has melted and the mixture is at a rolling boil, add the flour and quickly stir together with a wooden spoon until the mixture forms a dough.
With the pan still on a low heat, stir vigorously for 2 minutes, then tip the dough into a bowl and beat for a few minutes until it stops steaming. These two actions help to cook the flour and dry out the dough, which in turn helps it to absorb more egg. This helps the choux pastry to expand properly as it bakes.
Add the eggs, one at a time, beating until fully absorbed before adding the next. Depending on the flour used and how much water evaporated as you made the dough, the choux pastry will need varying amounts of egg, so the above is given as a guide.
With this recipe I usually add two eggs and then very slowly start adding the remaining egg, checking the texture of the dough after each addition. You are looking for a dough that has a shine and when it is lifted from the bowl, it should fall from the spatula in a ribbon that forms a "V" shape. If the dough doesn't contain enough egg, it won't expand properly and will be prone to cracking as it bakes; if there is too much egg, the dough won't hold its shape and will collapse as it bakes.
To prevent the dough from drying out and forming a skin, immediately put the dough into a piping bag with a 1.5cm plain round piping tip fitted.
Pipe blobs of choux pastry on the prepared baking trays that are around 2.5-3cm in size. Keep piping blobs that are 2cms apart until you run out of choux pastry. Depending on the size of your trays you may use all 3 trays or just 2 of them.
Top each choux pastry round with a craquelin disc and place the trays of choux pastry in the oven to cook for 30 minutes until the choux has risen and the craquelin is golden. You may find you need to turn the trays after 15 minutes to ensure the pastry colours evenly.
Turn off the oven and leave the choux pastry in the oven for 30 minutes to dry out.
To make the creme chantilly, in a large bowl combine the cream, vanilla and sugar. Whip until past the soft peak stage, but not quite stiff peak. Place the cream in a piping bag fitted with a 1cm round nozzle and return to the fridge until you are ready to use it.
Take the rhubarb curd and place it in a piping bag with a 1cm round nozzle fitted, place the curd in the fridge.
Pierce holes in the bottom of the choux buns using a sharp knife, make sure the hole is big enough for the nozzles to fit through. Place all the of the choux buns on a tray with the hole facing upwards.
In each choux bun pipe a small blob of rhubarb curd until you have filled every choux bun. You will not use all the curd, whatever you have left can be stored in a jar in the fridge for about 2 weeks at the longest.
Next take the creme chantilly and pipe it into all of the buns until they are completely full. Place the choux buns cream side down and when they are full start making the caramel.
Make sure you have all you need to hand, a cake stand or plate to make the croqembouche on.
The caramel will be the glue that keeps the whole gateau together. To make caramel, put half the sugar in a heavy based pan and melt it over a gentle heat. Add the rest of the sugar and melt, add the red food colouring until you get the colour you desire.
Take the caramel off the heat and leave it a few seconds to thicken. Dip the top of the choux puff in the caramel, you can place it in a silicone semi-sphere mould to set the caramel evenly. When cool, dip the side and bottom of the choux puff in the caramel and stick this to the bottom of the cake stand and create a circle of choux buns, keep dipping buns and work your way up to a point, creating smaller rings each time. I started with a 6 inch cake stand and built the croquembouche on there. Use the caramel like glue and keep working upwards. You will need to work quickly, but if the caramel hardens just warm it again to soften it.
You can decorate the croquembouche with spun sugar, tempered chocolate curls or even edible flower like violas or rose petals.