For the fudge, take every ingredient except the chocolate and the salt and add to a pan.
On high heat, combine those ingredients and bring to a rolling boil.
When the temperature reaches 115c, remove from the heat and allow to cool.
When the temperature cools to 50c, stir thoroughly and vigorously, then pour into any suitably wide tray. You want to make sure the chocolate can easily be sliced into bitesize pieces.
Leave in the fridge overnight to cool and harden, then cut into easily edible pieces.
For the chocolate, finely chop 100g into tiny pieces.
Add the other 200g to a pan on a very low heat and melt until it reaches 45 - 50c. Stir as you go, to make sure it melts evenly. You can use a bain marie at this stage, but it is not necessary.
Once there, add the rest of the chocolate back in, bit by bit, stirring it in thoroughly each time, until totally melted. Check the temperature each time. You want to reach exactly 29c. In other methods, you'll want to drop lower first and then bring the temperature back up, but you won't need to do that here.
Once you're at 29c and everything is thoroughly melted, start dipping the fudge into the chocolate and coating thoroughly. Drain what you can, then set aside on a movable flat surface like a chopping board.
Once all the buttery fudge chocolates are on the board, cool them for a few minutes in the fridge.
Once hard, remove them from the board, which might take a little poking and prodding, then you're ready.