These are crisp, round puffs of choux pastry with a crunchy, sweet craquelin topping and a filling of whipped banana cream and salted toffee sauce.
Make the craquelin. Beat together the butter and sugar until creamy. Mix in the flour, then gently knead to form a smooth, slightly sticky dough. Form into a ball, then place between 2 sheets of baking paper and roll out to a 3mm-thick disc. Transfer flat to the freezer.
Heat the oven to 180°C/160°C fan/350°F/Gas 4. Make the choux pastry. Put the milk, butter, salt and sugar with 65ml of water into a medium pan over a low heat.
Melt the butter, then turn up the heat to quickly bring to the boil. Remove the pan from the heat and tip in the sifted flour. Beat rapidly to a smooth, glossy paste.
Place the pan back on a low heat and stir for 1 minute, or until the dough leaves the sides of the pan clean. Tip into the bowl of a stand mixer fitted with the whisk, and leave to cool until barely warm.
Then, whisk in the eggs little by little, beating well after each addition, until you have a very shiny dough that holds its shape.
Transfer the choux dough to a large piping bag fitted with a 1.5cm plain nozzle. Lightly sprinkle or spray the lined baking sheet with water, then pipe 20 mounds of 4cm diameter onto the baking sheet, spaced well apart. Brush the tops with beaten egg to gently flatten.
Remove the craquelin from the freezer and use the 3cm cutter to stamp out 20 discs. Set one disc on top of each choux mound.
Bake for 25 minutes, then quickly open then close the oven door to release the steam. Reduce the temperature to 160°C/140°C fan/325°F/Gas 3 and bake the buns for a further 10 minutes, or until they are crisp and golden.
Transfer the buns to a wire rack, make a small hole in the side of each bun with a skewer to release the steam, and leave to cool.
Make the salted toffee sauce. Put the sugar and 40ml of water into a heavy-based, medium pan over a low heat. Gently melt the sugar, then increase the heat and bring the syrup to a boil and cook rapidly without stirring until it turns rich amber. Remove the pan from the heat.
Quickly but carefully stir in the butter, then the cream. Stir until smooth, then add the salt. Transfer to a heatproof bowl, leave to cool, then cover and chill until firm enough to pipe.
Make the banana Chantilly cream. Whip the cream to stiff peaks. Blitz the bananas, icing sugar and vanilla paste to a smooth, thick purée. Gently fold this into the whipped cream. Cover and chill.
To assemble the buns, transfer the Chantilly cream to the large piping bag fitted with a 7mm plain nozzle, and the toffee sauce to the small piping bag fitted with a 3mm nozzle or to the squeezy icing bottle.
Pipe the Chantilly cream into each bun through the steam hole, then push the tip of the small nozzle or icing bottle into the cream to add a small amount of toffee. Best eaten on the day.