To make the puff pastry, combine the flour, salt and 30g/1oz butter in a freestanding mixer with the paddle attached. Add 110ml/3¾fl oz water and the lemon juice and knead to a smooth dough. Cover with cling film and rest in the fridge for 15 minutes.
Take 2 large pieces of baking parchment and lay them either side of the remaining butter block. Flatten the butter using a rolling pin until it’s a square shape.
Once the dough has rested, roll out to a square large enough to envelope the butter. Place the butter in the middle of the dough at a 45 degree angle (so the corners of butter point to the edges of the dough). Wrap the butter in the dough and press to seal. Roll out into a long rectangle and fold the dough over itself into thirds (like folding a letter). Roll the dough out again then repeat the folding process. Cover with cling film and rest in the fridge for 15 minutes.
Repeat the rolling and folding process another 4 times. Rest the dough in the fridge for another 20 minutes.
Preheat the oven to 220C/200C Fan/Gas 7.
Once the dough has rested, roll it out to rectangle at least 45cm/17½in long and to a thickness of 5mm. Cut out long strips about 1½cm/3¾in thick and 45cm/17½in long.
Starting at the tip of a cream horn mould, wrap a strip of dough around in a spiral ensuring it overlaps. Repeat for all 12 moulds. Place on 2 baking sheets lined with non-stick baking paper. Bake for 15-20 minutes, until the pastry is crisp and golden-brown. Leave to cool a little before removing the moulds and placing on a wire rack to cool completely.
Melt the chocolate in a bowl suspended over a pan of simmering water then dip in the wide ends of the horns. Set aside to cool until the chocolate is set.
For the candied lime, bring a pan of water to the boil. Add the lime slices and boil for 2 minutes, then remove using a slotted spoon. Scrub the saucepan clean and rinse. Then replace the boiling water and lime slices and boil for 2 minutes. Repeat this process 4 times (this will remove any bitterness from the limes). After the final time, rinse out the pan and add the sugar and 100ml/3½fl oz water.
Add the lime slices and bring to a simmer. Cook for 20 minutes until the slices are translucent. Set aside to cool and then cut into quarters.
For the lime curd, mix the juice, zest, eggs, sugar and butter in a saucepan. Cook over a medium heat until smooth and thickened. Set aside to cool.
For the mascarpone Chantilly cream, whip the double cream to soft peaks, then whisk in the icing sugar. In a separate bowl, beat the mascarpone until smooth and then mix in 180g/6½oz of the curd. Fold the mascarpone mixture into the whipped cream. Spoon into a piping bag fitted with a 1cm/½in star nozzle.
To assemble the horns, place the pastry horns in mugs to help them stand upright. Fill them with the cream mixture and top with a piece of candied lime. Serve immediately.