Bring the milk, coconut milk and buttermilk to a simmer over low heat in a small saucepan. Keep warm while you prepare the caramel.
Combine the sugar and water in a medium saucepan over high heat and cook (do not stir) until it becomes a deep amber brown color. Slowly whisk in the warm milk mixture and continue whisking until smooth; add the vanilla seeds and corn syrup. Bring to a boil, reduce the heat to medium and cook, stirring occasionally with a wooden spoon, until the sauce is reduced by half and is the consistency of a caramel sauce, about 55 minutes.
Once the sauce is reduced, remove from the heat and whisk in the cold better, vanilla extract, salt and rum and whisk until combined. Transfer the sauce to a medium bowl and stir in the pecans and coconut. Let the frosting cool to room temperature, stirring occasionally before frosting the cake.