Preheat your grill to medium-high. Make the sauce by cooking the butter and brown sugar in a small pot over medium heat, stirring occasionally, until the sugar melts. Stir in the whipping cream and vanilla. Bring to a boil for 30 seconds, stirring occasionally, and then remove from heat and stir in the cashews. Set aside until the pineapple rings are ready. Brush the pineapple rings lightly with the oil. Grill 1 to 2 minute per side, or until nicely coloured and heated through. Set on dessert plates. Divide and spoon the sauce on top. Garnish with mint and serve.