In a heavy saucepan, stir the sugar and wate together over medium high heat, until the sugar has dissolved and it's clear. Do not let the mixture boil yet.

Once it's clear and the sugar has dissolved, turn the heat up to high, cover and allow to heat to boil for 2 minutes. (begin counting when you put the lid on, not when it boils)

After the 2 minutes, uncover the saucepan and allow to boil, swirling the pan around occasionally, you see the mixture get dark around the edges.

As it begins to darken, begin swirling the pan continuously until the syrup is a deep amber color and just begins to smoke.

Remove immediately.

Add the butter, and mix until melted and mixed in.

Next, stir in the heavy cream.

When adding in the heavy cream, if the sauce starts becoming lumpy, place back over very LOW heat while stirring until smooth. One smooth remove immediately from heat.

Now it's time to add the finely chopped bittersweet chocolate and stir until it is fully melted.

Stir in the vanilla and the fleur de sel (or sea salt flakes).

Pour finished chocolate sauce into mason jars, seal and tie a tag on.