For the crumble topping, mix the flour and sugar together in a medium bowl. Add the butter and rub until you have a breadcrumb texture. Using your hands, press this mixture together to form a uniform dough. Place this between 2 sheets of baking parchment and roll out until it is a few millimetres thick. Pop in the freezer on a baking tray until needed.
For the salted caramel, put the sugar in a medium-sized saucepan and set over a medium heat. Allow the sugar to melt and caramelise, stirring occasionally. Once the sugar is a dark copper caramel colour, add half the double cream. When the bubbling has subsided, add the remaining cream, the butter and salt, and stir to combine. If you have any remaining lumps, put back on the heat and stir until melted and smooth. Remove from the heat, pour into a small bowl, cover with cling film and chill until needed.
For the popcorn cream, put all the ingredients in a medium saucepan over a medium-high heat and bring to the boil. Remove from the heat, cover the pan and allow to infuse for 1 hour. Strain the cream through a sieve into a measuring jug, pressing down on the softened popcorn to get as much cream out as possible. Top the cream up so you still have 600ml. Press a piece of cling film onto the surface of the cream and chill until needed. Can be prepared the day before up to this point.
For the choux pastry, heat oven to 180C/160C fan/gas 4 and line 2 baking trays with baking parchment. Using a 5cm cookie cutter as a guide, draw 5 circles onto each piece of baking parchment and turn them over. Put 140ml water, the butter, ¼ tsp salt and the sugar in a medium saucepan and, over a medium-high heat, bring the mixture to a full rolling boil. Once boiling, and the butter has melted, remove from the heat, tip in the flour and immediately mix together with a wooden spoon until a smooth paste forms. Return to the heat and beat for 1-2 mins to dry out a little. Tip the paste into a large bowl and beat for a few mins to cool down slightly.
Add in the eggs, one at a time, beating until fully combined before adding another – add the last egg a little at a time as you may not need it all. Stop when the smooth choux drops off your spoon in a V-shape. Put the dough in a piping bag fitted with a large round nozzle, then pipe rounds of dough onto the prepared baking trays, using the circles as templates. Remove the crumble topping from the freezer and peel off the top layer of parchment. Using a 5cm cookie cutter, cut out 10 discs of the crumble topping and place one on top of each round of dough. Bake for 40 mins, or until the choux is well risen and the crumble is golden. Turn the oven off but leave the choux in until fully cooled.
To assemble the buns, use a large serrated knife to carefully slice in half. Place a few pieces of popcorn in the base of each bun and spoon some caramel in too (if you have prepared the caramel ahead of time, it will have firmed up, so place over a bowl of simmering water to loosen it). For the filling, remove the cream from the fridge and whip to soft peaks, then put in a piping bag fitted with a large star tip and pipe a swirl onto the base of each bun. To finish, place the top of the choux buns back on top of the cream and dust lightly with icing sugar. I finished mine with a piece of edible gold leaf.