Pour maple syrup in a deep pot with a thick bottom. Being to a boil over medium heat and let boil softly for ~15 minutes. Stir every few minutes with a heat proof spatula or spoon to reduce the bubbles. You will notice that the syrup will start to coat the outside of the spatula or spoon and will cool quickly into a caramel-like consistency. It will resemble candy at the softball stage.
Stir in the butter until it is completely melted and then add the cream, stirring constantly. It will not bubble up dangerously like caramel but it will still be very hot. Add the salt and then stir to mix.