The tang of fresh lemon and rosemary gives these biscuit bars a taste of the Mediterranean.
Put the rosemary in a medium bowl along with the sugar, lemon zest and both flours. Melt the butter in a small pan over a low heat. Pour it gradually into the bowl, using a wooden spoon to mix everything together to a firm dough. Divide the dough in half, flatten each half into a disc and wrap in cling film. Chill while you make the curd.
Place the eggs, sugar, lemon juice and zest and butter in a heatproof bowl set over a pan of gently simmering water. Stir continuously with a wooden spoon for about 10 minutes, until thickened. Pour the curd into a lined baking tray and chill until thick and spreadable.
Heat the oven to 180°C/160°C fan/350°F/Gas 4. Roll out the chilled dough on a lightly floured work surface to a rough rectangle about 6mm thick. Using a sharp knife, cut out 12 rectangles, each measuring 8 x 3.5cm. Place the biscuits on a lined baking tray and bake for 12–15 minutes, until the edges are turning golden. Remove from the heat, trim the edges to fit the moulds and leave on the tray to cool completely.
Temper the chocolate for the shells. Melt 350g of the chocolate in a heatproof bowl set over a pan of gently simmering water until it reaches 55°C/131°F on the sugar thermometer. Remove one third of the melted chocolate to a separate bowl, and put the remaining 100g solid chocolate into the remainder over the pan of water. Stir it in and cool the chocolate to 28°C/82°F, then add back in the small amount of melted chocolate and mix until the chocolate reaches 30°C/86°F. Remove from the heat and, using the pastry brush, use the tempered chocolate to coat the insides of the 12 bar moulds, then chill to set.
Make the marshmallow. Pour 4 tablespoons of water into a shallow bowl, add the gelatine leaves and leave to soften for 5 minutes. Put the egg white and the 1 tablespoon sugar in a stand mixer fitted with a whisk.
Place the 150g of sugar, along with the glucose and 35ml of water in a medium pan over a medium heat. When the sugar reaches about 116°C/241°F on the thermometer, begin whisking the egg whites to soft peaks. Keep an eye on the temperature of the sugar and when it reaches 120°C/248°F, remove the sugar immediately from the heat.
Squeeze out the gelatine and add to the pan with the melted sugar. Immediately 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 meringue is thick and glossy and forms stiff peaks. Add the lemon extract, then transfer the mixture to the piping bag fitted with the medium round nozzle. Leave to cool for 10 minutes, or until thick enough to pipe and hold its shape. Then, pipe a 1cm layer of marshmallow into the base of the moulds and leave to set. (You will have lots of marshmallow left over – see the tip below for how to use it.)
Spread a little lemon curd over the marshmallow, then press a biscuit on top of each. Try to make the biscuits level with the top of the mould.
Spoon the remaining tempered chocolate (you may need to re-melt it) over the top of the mould tray to cover the biscuits, then scrape off the excess with a palette knife, evening out the tops to complete the chocolate shells. Chill for 1–2 hours, until set, and turn the bars out of the moulds.
To decorate, mix together the icing sugar and gin to a thick, piping consistency. Spoon the icing into the piping bag fitted with the small round nozzle and pipe diagonal lines over the corner of each bar. Decorate with a sprig of rosemary to finish.
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.