Method

• 1.
  For the blackberry and lime jam, place blackberries and sugar in a small saucepan over low heat. Cook, stirring occasionally, for 30-40 minutes until sugar dissolves and jam is thick and glossy. Stir in lime juice and cook for a further 5 minutes or until a small amount of jam sets when spooned onto a cold plate. Set aside to cool.
• 2.
  To make the shortbread, place butter, lemon zest and icing sugar in the bowl of a stand mixer with the paddle attachment and beat for 6-8 minutes until soft and pale. Add flour, egg yolks and a pinch of salt flakes and beat to form a soft dough. Turn dough onto a flat surface and shape into a disc. Cover and chill for 30 minutes.
• 3.
  Preheat oven to 170°C. Line three large baking trays with baking paper. Halve dough, then roll out each half between 2 sheets of baking paper to 1cm thick. Use a 6cm round cutter to cut biscuits from dough and place on prepared trays. Chill for 30 minutes (this will ensure they hold a nice round shape during cooking).
• 4.
  Bake, in batches if needed, for 13-15 minutes until lightly browned on the bottom. Cool on tray for 5 minutes, then transfer to a wire rack to cool completely.
• 5.
  Meanwhile, to make the buttercream, place sugar and 65ml water in a small saucepan over low heat and bring to 114°C.
• 6.
  Meanwhile, whisk egg whites in a heatproof bowl to soft peaks. Gradually whisk in hot sugar syrup in a slow, steady stream, whisking continuously until whites are thick, glossy and completely cool.
• 7.
  Slowly add one-third of the butter, whisking until combined. Repeat with remaining butter in two batches, adding lime zest with the final batch, whisking until buttercream thickens.
• 8.
  Spoon buttercream into a piping bag fitted with a 1cm plain nozzle. Pipe buttercream around the edge of half the shortbread. Fill centre with jam. Sandwich with remaining shortbread. Serve.