Line a standard 1lb loaf tin with clingfilm, ensure there's plenty of overlap.
Add the cream to a saucepan and bring it to the boil, in another bowl add the chocolate, chop it into small pieces if necessary.
Combine the cream with the chocolate and stir until all of the chocolate has melted, then add the fleur de sel and stir until dissolved.
Pour the ganache into the prepared tin and lightly fold the cling-film over to cover it. Place the ganache in the fridge and leave it to set for a couple of hours.
Take the dark chocolate and melt it gently over a bain-marie, heat the chocolate to around 45C, remove the chocolate from the heat even if all the chocolate hasn't melted, set the chocolate on a tea-towel and stir it until the chocolate has melted.
Take the ganache from the fridge and peel off the cling-film, at this stage leave the ganache as a slab.
Pour half the melted chocolate over the ganache and dust it with cocoa powder. Leave the chocolate to set for a minute or two and then flip it over on the cling-film.
Coat the top of the ganache with the remaining chocolate and sprinkle it lightly with the fleur de sel.
Leave the chocolate to set before trimming the edges of the ganache and then cutting into squares, diamonds or rectangles. The simplest way to do this is to warm the knife in water and wipe it before each cut.
Return the ganache to the fridge and allow it to cool for another 30 minutes. The ganache squares will keep in an airtight container for up to 2 weeks, if it's humid, you'll need to store them in the fridge, but remove them about 10 minutes before eating them.