Preheat the oven to 200C/180C Fan/Gas 6.
Place the butter, milk and sugar in a saucepan, pour over 330ml/11½fl oz water and add a pinch of salt. Slowly bring to the boil, then lower the heat and simmer until the butter has melted. Sift and stir in the flour until the mixture comes away from the sides and forms a ball. Beat for 30 seconds, then set aside to cool.
Beat the eggs into the mixture one at a time using an electric or manual whisk, until glossy.
Using a piping bag with a 1–1½cm/½–⅝in nozzle, pipe 3cm/1¼in diameter rounds on three to four baking trays (or cook in batches). Bake for 10 minutes, then turn down the heat to 180C/160C Fan/Gas 4 and cook for 20–25 minutes. Make a small slit in the side of each bun, then bake for another 5 minutes.
At this point you can continue with the recipe, store in an airtight container or freeze until ready to use (if doing the latter, reheat the buns in the oven for a few minutes before cooling and filling).
To make the spiced cream, whip the cream until quite stiff. Sift and stir in the icing sugar with the coffee extract and cardamom. Cover with cling film and refrigerate for 30 minutes. Using a piping bag, pipe the cream into the slit in each bun. Wipe away any excess and refrigerate the buns until ready to assemble.
To make the caramel, place the sugar in a heavy-bottomed saucepan over a medium heat. Heat until 154C when all the sugar will have dissolved and caramelised. (Move the saucepan around if it looks like one edge is darkening before the other and use a brush dipped in water to wipe away any crystals that form on the sides.)
Dip the top of the buns into the caramel, then sprinkle with sugar nibs. Repeat with as many of the buns as you want decorated.
To assemble, dip either side of the buns in the caramel. One side will stick to the bottom, one side will glue to the top (the sugar nibs will face outwards). Continue assembling the buns until you reach the top of the cone. If the caramel becomes too sticky to work with, place over a low heat to warm.
Leave to set completely, then remove the cone. Indoor sparklers will add extra flare.