Use a spoon to scoop out the very thick, creamy coconut cream at the top of the can of coconut milk, and place in a small saucepan. Cook over medium heat, stirring occasionally, and bring to a simmer. Stir in the sugar, vanilla, and a tiny pinch of salt. Reduce heat to low. Stir in the arrowroot mixture, and continue to cook 5-10 minutes, stirring frequently, until the mixture is thick enough to coat a spoon. If the sauce seems too thin, add another teaspoon of arrowroot mixed with 1 tablespoon of water to the pot and cook another few minutes. Remove from heat and let the saucepan cool, then place in the fridge to chill completely.