Directions

• For the biscuits:
• Cream together the butter and sugar in a large mixing bowl until light and fluffy.
• Add the beaten egg to the bowl and beat until combined.
• Add the lime zest, lime juice and mint to the mixing bowl and beat them in until mostly combined. The mixture may start to split at this point and you won't be able to get all of the juice to combine, but don't panic, it will all be fine once we've added the flour!
• Sift the flours and salt into the bowl and gently beat the mixture until it comes together to form a soft dough.
• Using your hands, gently form the dough into flat disc, wrap in cling film and place in the fridge for at least 30 mins, or until the dough feels firm enough to roll without sticking.
• Roll the dough out onto a lightly floured surface until it is approximately 5mm thick. Using a 5.5cm round cutter, cut as many biscuits out of this dough as you can, before using a pallete knife to pick up and place each of the rounds onto a lined baking tray.
• Reroll any dough you have remaining once more and repeat the process of cutting out rounds and placing them onto a baking tray.
• Place each baking tray into the fridge for at least 15 mins or until the biscuit rounds are firm and easy to handle without becoming misshapen.
• Preheat the oven to 180C / 160C fan / Gas Mark 4.
• Take one tray of biscuits from the fridge and place it onto the middle shelf of the oven to bake for 6 mins. Then, gently remove the tray, rotate it 180 degrees and place it back into the oven for 3-4 mins to allow the biscuits to bake evenly.
  The biscuits shouldn't colour too much, so if they begin to brown on the edges, remove them from the oven to prevent burning.
• Leave the biscuits to cool on the tray for 10 mins, before transferring to a wire rack to cool completely.
• Repeat Steps 9 and 10 for each of the remaining trays, until all of your biscuits are baked!
• For the curd:
• Weigh the butter, sugar, lime zest and lime juice into a small saucepan and place it over a low heat, stirring occasionally until the butter melts.
• Whisking constantly, pour the beaten egg and mint into the pan and continue to whisk until the curd thickens enough to coat the back of the spoon.
• Pour the curd through a sieve into a heatproof bowl, gently stirring the contents of the sieve to ease the curd through. Allow the curd to cool slightly at room temperature, before placing in the fridge to chill.
• For the buttercream:
• Beat the butter in a large mixing bowl until light, fluffy and significantly paler in colour.
• Sift half of the icing sugar into the bowl and beat until fully combined into the butter. Repeat this for the remaining half of the icing sugar.
• Add the rum to the bowl and beat until combined.
• Place the buttercream into a piping bag until you are ready to fill the biscuits.
• Assembling the biscuits:
• Pair up your biscuits so that you have pairs of roughly equal size and shape.
• For each pair, pipe a ring of buttercream onto the bottom of one of the biscuits, then spoon a little of the curd into the centre of this buttercream circle. Place the other biscuit on top of this to create a sandwich biscuit!