If preparing rum raisins, begin by soaking the raisins in hot water (just enough to cover raisins), add the rum and set aside.
In a large oven-proof skillet over medium heat, add the butter and brown sugar. Stir until the butter is melted and the brown sugar dissolves. Nestle the pear halves, cut-side down, into the melted butter and brown sugar, and brush or spoon some of the mixture over the pears. Roast the pears in the oven until tender and can easily be pierced with a fork, about 15 minutes or longer depending on the ripeness of the pears.
Transfer the pears to a serving plate, cut-side up, and spoon butter and brown sugar mixture over the cooked pears.
Add a dollop of vanilla ice cream to the hollowed cores letting some of the cool cream melt down and around the caramelized pears, top with an optional spoonful of rum raisins that have been strained. Serve immediately.