In a small pot, over medium-low heat, whisk together the coconut milk and sugar. Once the sugar has dissolved lower the heat, and let it simmer for 3-5 minutes.
Add the butter or coconut oil, and continue to cook over medium-low heat, until it starts to thicken, about 15 minutes.
Remove from heat, and add in the vanilla, cardamom, and salt. The mixture will continue to thicken as it cools.
Notes
This recipe makes about 1 cup of sauce. Since it's pretty rich I figured 1 tablespoon per serving.