This twist on a traditional walnut whip hides delicious coffee ganache in a bed of fluffy marshmallow all wrapped in a crisp chocolate shell.
Make the biscuit. Heat the oven to 200°C/180°C fan/400°F/Gas 6. Tip the walnuts onto a baking tray and toast for 3–5 minutes, until they are tinged a darker shade of brown. Remove from the oven and leave to cool. Reduce the oven temperature to 160°C/140°C fan/315°F/Gas 3.
Place the walnuts in the bowl of a mini food processor and blitz until ground. Sift the flour and baking powder together into a bowl.
In a separate bowl, cream the butter and icing sugar together until light and fluffy, then add the egg yolk and stir until combined. Fold in the ground walnuts, then the flour and baking powder to form a dough.
Form the dough into a ball, press into a disc, wrap in cling film and chill for 15 minutes, until firm enough to roll.
Meanwhile, begin the coffee ganache. Pour the cream into a small pan and add the coffee beans. Place over a medium heat and bring to just below boiling point, then remove from the heat and leave to infuse for 15 minutes.
While the cream is infusing, tip out the chilled dough onto a lightly floured surface and roll it out until 3mm thick. Using the 4cm round cutter, stamp out 8 discs and place them on a baking sheet. Chill for 10 minutes, then bake for 20 minutes, until golden. Leave to cool on the baking sheet for 5 minutes, then transfer to a wire rack to cool completely.
Heat the coffee and cream back up to just below boiling point. Place the chocolate in a bowl, then pour the hot cream through a sieve over the chocolate. Discard the coffee beans. Leave the chocolate to melt for 3 minutes, then stir to a smooth ganache. Leave to cool until firm, then transfer to the piping bag fitted with the 1.5cm plain nozzle.
For the marshmallow, soak the gelatine in a bowl of cold water for 5 minutes, until soft. Place the egg whites and cream of tartar in the bowl of a stand mixer fitted with the whisk and whisk to firm peaks.
Tip the sugar and corn syrup into a clean pan, place over a medium heat and bring to the boil. Boil until the temperature on the sugar thermometer reaches 115°C/239°F.
Immediately remove the syrup from the heat and, with the mixer on a slow speed, pour the hot sugar down the sides of the egg whites, whisking all the time. Increase the speed to medium–high and whisk for 3–5 minutes to a thick meringue.
Drain the gelatine, squeezing out any excess water with your hands, then roughly chop into small pieces. Add the gelatine a few pieces at a time to the meringue, whisking continuously between each addition, until all the gelatine has been incorporated and the marshmallow is stiff. Whisk in the vanilla paste.
Spoon the marshmallow into a piping bag fitted with the 1cm plain nozzle. Secure the end of the piping bag and chill for 15 minutes.
To assemble the walnut whips, pipe a kiss of coffee ganache about 1.5cm wide by 2.5cm tall in the centre of each sable biscuit.
Pipe the marshmallow in a spiral around the coffee ganache to encase it. Chill to set for 10–15 minutes.
Make the coating. Melt 225g of the chocolate in a bowl set over a pan of gently simmering water until it reaches 44°C/111°F on the thermometer. Remove the bowl from the heat and add the remaining 75g of chocolate, stirring until the chocolate has melted and cooled to 32°C/90°F .
Place the walnut whips on a cooling rack set over a baking tray and pour the tempered chocolate evenly over them to coat. Top each walnut whip with a walnut half and leave to set.