Sift flour and salt into a bowl and break cold butter into flakes within the mixture.
Add 125ml ice-cold water, knead the dough briefly, and chill in the freezer for 15 minutes.
Roll dough into a 4mm thick rectangle, fold into thirds, turn 90 degrees, repeat, and chill for 10 minutes.
Repeat the rolling and folding process twice more and chill in the freezer for 15 minutes.
Preheat the oven to 220°C.
Roll dough to 3mm thick and cut into twelve 2cm by 30cm strips.
Wrap strips around moulds with overlapping edges, place on a silicone mat, and chill for 5 minutes.
Brush with beaten egg, sprinkle with sugar, and bake for 10 to 12 minutes until golden.
Cool on a wire rack after removing the moulds.
Beat mascarpone until soft and mix in sifted icing sugar.
Whip double cream to soft peaks and fold into the mascarpone with the coffee paste.
Melt the dark chocolate and place chopped nuts in a separate bowl.
Brush the inside of cooled horns with chocolate and dip the rims into chocolate and then hazelnuts.
Pipe the coffee cream into the horns, dust with icing sugar, and serve.