Heat a heavy bottomed saucepan over medium heat and melt the butter, dark demerara sugar and vanilla extract. Stir until melted and the sugar has dissolved.
Add the condensed coconut milk and a good pinch of salt and mix well. Bring the mixture to a boil, stirring repeatedly.
Remove from the heat and transfer into a jar. The mixture will be very hot, so make sure you leave it to cool for a few minutes!