To make the dough, sieve the flour and 1 big pinch of fine sea salt onto a clean work surface, then make a well in the middle.
Combine the yeast, sugar and 650ml of tepid water in a jug, leave for a few minutes until it starts to froth, then pour into the well.
Using a fork, gradually bring the flour in from the sides and swirl it into the liquid. Keep mixing and incorporating the flour until it comes together into a rough dough. With clean, flour-dusted hands, work the rest of the flour into the dough, then knead until it forms a smooth, springy and slightly wet dough.
Place into a large flour-dusted bowl, dust with a little more flour, then cover the bowl with a damp cloth. Leave in a warm place for about 1 hour, or until doubled in size.
To make the filling, spread the hazelnuts out on a baking tray, then place in the oven for 10 minutes, or until golden. Leave to cool completely.
Reserving a small handful of the toasted nuts, place the rest into a food processor with the sugar, then blitz to a fairly smooth paste.
Snap the chocolate into a medium heatproof bowl, and rest over a pan of simmering water on a medium heat. Stir until melted.
Add the cream and 100g of butter, then stir until smooth and combined. Fold in 1 big pinch of sea salt and the hazelnut paste.
Knock back the risen dough, then divide into 14 portions. On a flour-dusted surface, roll each portion out into rough 16cm rounds.
Spoon 1 heaped tablespoon of the chocolate mixture onto one side of each pastry round, then fold the pastry over the filling into a half-moon shape, crimping the edges to seal – they should look like Cornish pasties.
Melt a small knob of the remaining butter in a large frying pan on a medium-low heat, then add as many calzones as will fit, being careful not to break or pierce them. Cook for 8 to 10 minutes, or until golden and crisp, turning halfway.
Roughly chop or bash the reserved toasted nuts.
Slice the calzones into wedges or halves, if you like, then serve straightaway with a few toasted hazelnuts scattered on top. If you have any leftover chocolate, melt it and drizzle on top. Repeat with the remaining calzones, serving as you go.