Grill each pineapple slice over medium heat on your barbecue. (a grill pan indoors on your stove can be used also) Sear each side with the lid open on your grill for about 3-5 minutes each. You want the pineapple to be cooked but not too soft. This way it will hold up to the rest of the sundae ingredients.
Place the cooked pineapple slices in the refrigerator while you make the rum sauce. This way they will be cool and not melt your ice cream once you start assembling.
Melt the butter in a small saucepan over medium-high heat. Add the brown sugar and whisk to dissolve. Add the rum and vanilla and whisk to combine. Continue to cook over medium-high heat and stir constantly.
Continue to stir and cook about 7-10 minutes. This will bubble and start to thicken. Cook the sauce until it is reduced by about half. It is ready when you dip a spoon into the sauce and it coats the back of the spoon. If you let it cool a few seconds and then drag your finger carefully down the back of the spoon, a trail should be left in the middle of it if it is thick enough. Remove from heat.
To assemble the sundaes, place a slice of pineapple on a plate. Top with a scoop of ice cream and then drizzle on the rum raisin sauce. Add whipped cream and additional raisins if desired.