Put the chocolate into a bowl, snap it up into squares or chop it into smaller pieces if necessary.
Place the hazelnut paste, brown sugar, whipping cream and fleur de sel in a saucepan and bring to a gentle boil. Stir regularly to make sure the hazelnut paste is mixed in with the cream and that there aren't any lumps.
Add the hot cream mixture to the chocolate and stir until it has all melted and mixed together.
Cover with clingfilm, pressing it against the ganache so that you don't get any condensation and place the ganache in the fridge to set for at least 4 hours.
Take teaspoonfuls of the ganache and roll them into balls. Coat the truffles in cocoa powder and return them to the fridge, you should end up with around 20-25 truffles.
Next temper your chocolate, take 200g of the chocolate, place it in a bowl over a pan of barely simmering water. Melt the chocolate to 45C - this will take a bit of time and you will find you have some pieces of chocolate that haven't fully melted. Still remove the bowl from the heat as the rest of the chocolate will melt. Don't be tempted to heat it to more than 45C.
Add the remaining 100g of chocolate to the melted chocolate and stir until it all melts. You might need to add more chocolate, you want to lower the temperature to 29C, then place the bowl back on the pan and use the residual heat to raise the chocolate temperature to 32C.
You can now coat the truffles in the tempered chocolate, to coat them use your fingers and swirl them around in the chocolate and allow the excess to drip off. Place them on a piece of greaseproof paper or on a silicone mat to set.
If you want, you can dust these with gold dust.
If you cover the truffles in tempered chocolate, they can be stored outside of the fridge in an airtight container for a couple of weeks. If you only cover them in cocoa, store them in the fridge and eat within two weeks.