In a saucepan, over medium heat, reduce the maple syrup to about 500 ml (2 cups).
Add the maple sugar and cook without stirring until obtaining a clear caramel.
Meanwhile, in a small saucepan or microwavable container, heat the cream.
Remove the saucepan of caramel from the heat and gradually add the warm cream, taking care not to splash. Return to the heat, place a candy thermometer in the saucepan and simmer the caramel until the thermometer reads 112°C (233°F).
Add the chocolate and fleur de sel, stirring well to achieve a homogenous consistency.
Add the butter with the mixer at low speed and, off the heat, incorporate it well with a stick blender until obtaining a nice smooth caramel.
Strain through cheesecloth and place in containers.