In a medium saucepan heat the sugars with the fleur de sel on a gentle heat. Do not stir but shake the pan around occasionally so the sugars melt evenly. Be careful as the sugars can burn quickly.
When just melted and turning a golden brown remove from the heat, then pour in the whipping cream. It will bubble up a lot so be cautious.
Then add the butter and keep on stirring.
Once all the butter has melted stir in the bourbon until it has all mixed together evenly.
Leave to cool for a couple of hours whereupon the bourbon will mellow out and the caramel will thicken slightly.
Meanwhile you can be tempering your chocolate as described here.
Then coat your moulds with chocolate as described above, leave to set for about an hour.
Fill your chocolate moulds with the thickened caramel, making sure not to overfill.
Then you can straightaway pour some more tempered chocolate over the top of the mould, scrape a palette knife over the surface of the mould to remove the excess chocolate then leave to set for about 1 hour.
Turn the moulds upside down and the chocolates should happily drop out.