Preheat the oven to 400°. Generously butter a 9-inch-round or 8-inch-square nonreactive baking dish. Toss the plums with the sugar and rum; arrange in the prepared dish in an even layer. With a rolling pin, crush the toasts in a bag to 1/4 -inch crumbs. (Or pulse in a food processor.) Toss the crumbs with the butter; scatter over the fruit. Bake for about 15 minutes until the plums are soft and the top is golden. Serve warm or at room temperature over ice cream.