These lime shortbreads are topped with a fresh mint marshmallow and lime jelly, enrobed in white chocolate. Divine!
Make the shortbread. Beat the butter and sugar in a stand mixer fitted with the beater on medium speed for about 5 minutes, until pale and creamy. Mix in the flour, salt, lime zest and enough lime juice to form a stiff dough. Shape into a 3cm-thick square, flatten, wrap in cling film and freeze for 10 minutes.
Roll out the chilled dough on a lightly floured work surface to a rough rectangle 6mm thick. Using the cutter or template, cut out 24 biscuits and lay them on the lined baking sheets. Chill for 10 minutes.
While the shortbread is chilling, heat the oven to 180°C/160°C fan/350°F/Gas 4. Make the mint sugar. Place the caster sugar and mint leaves into a mini chopper or food processor and blitz to a fine powder, then set aside.
Make the candied mint leaves. Put 50ml of water and 50g of the caster sugar in a frying pan over a low heat and bring to a simmer. Place the remaining sugar on a plate and set aside. Taking three mint leaves at a time, dip the leaves into the syrup in the frying pan, turning after 30 seconds and removing with a fork after a further 30 seconds. Drop each coated leaf onto the plate of sugar, turn to coat, then lay the leaves on a wire rack to dry out for about 1 hour.
Meanwhile, prick the chilled shortbread all over with a fork and bake for 12 minutes, checking and turning the trays after 6 minutes to ensure an even bake, until the shortbreads are just golden. Remove from the oven and allow to cool completely on the baking sheets.
Make the jelly. Pour 4 tablespoons of water into a shallow bowl, add the gelatine and leave to soften for 10 minutes. Meanwhile, put the lime juice and zest in a small pan with the sugar and 30ml of water. Place over a low heat, stirring until the sugar dissolves. Then, bring the syrup to the boil and heat until it reaches 105°C/221°F on the sugar thermometer. Remove from heat.
Squeeze out the gelatine and add to the pan with the syrup. Stir in a drop of green food colouring, then pour the mixture into a shallow container and allow to cool at room temperature until set (about 20 minutes).
Make the marshmallow filling. Pour 100ml of water into a small bowl, sprinkle over the gelatine and set aside. Place the egg white in the clean, grease-free bowl of a stand mixer fitted with the whisk and whisk to stiff peaks, continue whisking adding the mint sugar a tablespoonful at a time.
Pour 150ml of water into a small pan over a low heat and add the caster sugar and corn or golden syrup. Stir until the sugar dissolves, then increase the heat and bring to the boil until the syrup reaches 120°C/248°F on the sugar thermometer. Remove the pan immediately from the heat, stir in the gelatine, then pour the sugar syrup in a thin stream down the side of the mixer bowl into the egg whites, whisking for about 6 minutes, or until the mixture forms stiff peaks. Spoon half the mixture into the large piping bag and set aside.
To assemble, spread a thick layer of jelly onto 12 shortbread biscuits. Snip a 1cm hole in the large piping bag and pipe a test strip of marshmallow on a piece of baking paper, making sure it is firm enough to hold its shape – if not, leave it to set a little longer. Then, pipe a thick layer of marshmallow on top of the jam on each shortbread. Place a second biscuit on top to form 12 biscuit sandwiches and chill for 30 minutes to set. (You will have some marshmallow left – see the tip below for how to use it.)
Once the biscuits have chilled, make the shells. Melt the white chocolate in a heatproof bowl set over a pan of gently simmering water. Do the same with the dark chocolate, in a second bowl set over a pan of gently simmering water.
Once the chocolates have melted, hold one sandwiched biscuit between your finger and thumb and dip to coat in the white chocolate. Shake off the excess and transfer to a wire rack set over a sheet of baking paper. Spoon over a little white chocolate to fill any gaps. Repeat for the remaining biscuits.
Place the dark chocolate in the small piping bag. Snip the end and pipe fine stripes across the width of each biscuit. Drag a cocktail stick through the lines along the length of the biscuits to create a feather pattern. Top with a candied mint leaf and chill for at least 15 minutes to set before serving.
Tip: To use up the leftover marshmallow, dust a 20cm square shallow cake tin with 2 tablespoons of icing sugar and 2 tablespoons of cornflour. Pour the remaining marshmallow on top, then dust with another 2 tablespoons of icing sugar and 2 tablespoons of cornflour. Leave to set, then cut into squares and serve as sweets.