Sieve the flour and ¼ tsp salt into a bowl. Bring the butter and 225ml water to the boil in a saucepan, then simmer until the butter has melted. Tip in the flour mixture and quickly beat with a wooden spoon until everything comes together into a smooth, shiny dough that pulls away from the side of the pan. Tip the dough into a clean bowl, and spread it up the side with the spoon to help it quickly cool down a bit (you don’t want it to cool completely – it’s easier to incorporate the eggs while it’s still a bit warm). Heat the oven to 200C/180C fan/gas 6 and line two baking sheets with baking parchment. Set aside.
While the dough is still slightly warm, crack in the eggs one at a time, beating well with the wooden spoon between each addition until smooth (you may not need to use all the eggs). When ready, the dough should slowly fall off the spoon in a V-shape – if the mixture is too runny, you won’t be able to pipe it; if it’s too thick, it won’t puff up in the oven. Spoon the dough into a piping bag and snip off the end so it has a 1cm opening.
Holding the piping bag at a 90-degree angle to the baking tray, pipe walnut-sized balls of dough over the two sheets – you should get 18-20 in total. Dip your finger in a little water and gently pat down any peaks on top of the balls. Bake for 30-35 mins, swapping the trays over after 20 mins. The choux buns should be puffed up and crisp when cooked. Leave to cool completely.
To make the filling, beat the cream with the biscuit spread and spices in a large bowl using an electric whisk until softly whipped.
For the caramel sauce, tip the sugar into a saucepan set over a medium heat, add 6 tbsp water and cook until the sugar has dissolved. Turn up the heat slightly and simmer, swirling the pan occasionally until golden (do not stir or the sugar will crystallise). Remove from the heat, add the butter and swirl the pan until the butter has melted and combined with the syrup. Quickly beat in the cream, vanilla, cinnamon, mixed spice and a pinch of sea salt. Leave to cool slightly.
Cut each cooled choux bun in half through the middle. Spoon the filling into a piping bag, snip off the end and pipe over the bases of the buns. Sandwich these with the tops, then arrange in a roughly 27cm round wreath shape on a large serving plate or board. Drizzle all over with the caramel sauce, then scatter over the crumbled biscuits and gold sprinkles, if using. Best eaten straightaway. Will keep chilled for up to 24 hrs.