Preheat your grill to medium-high heat.
In a saucepan over medium heat, melt the butter. Stir in brown sugar, coconut milk, coconut rum, and salt. Bring to a simmer and cook for 3–4 minutes until slightly thickened. Remove from heat and set aside.
Place pineapple slices on the grill. Cook for 2–3 minutes per side or until grill marks appear and the pineapple caramelizes.
Serve warm pineapple drizzled with coconut rum sauce. Add vanilla ice cream or toasted coconut on top if desired.