Combine flour, salt, 30g butter, 110ml water, and lemon juice into a smooth dough.
Rest the dough in the fridge for 15 minutes.
Flatten the remaining butter block into a square between sheets of baking parchment.
Envelope the butter square in the rolled-out dough and seal the edges.
Roll the dough into a rectangle, fold into thirds, and repeat the folding process.
Chill for 15 minutes, repeat rolling/folding four times, then chill for 20 minutes.
Preheat your oven to 220°C.
Roll dough to 5mm thickness and cut into 45cm long strips.
Spiral pastry strips onto 12 moulds and bake on lined sheets for 15-20 minutes.
Cool pastry slightly, remove moulds, and cool completely on a wire rack.
Melt dark chocolate and dip the wide ends of the cool pastry horns.
Boil lime slices in water four times to remove bitterness, changing water each time.
Simmer lime slices in a 100ml water and 100g sugar syrup for 20 minutes.
Cool candied limes and cut them into quarters.
Cook lime juice, zest, eggs, sugar, and butter in a pan until thickened.
Whip double cream with icing sugar to soft peaks.
Beat mascarpone until smooth and mix in 180g of cooled lime curd.
Fold the mascarpone mixture into the whipped cream and transfer to a piping bag.
Fill the pastry horns with cream and top with a candied lime quarter.