Preheat broiler. Place plum halves, cut sides up, in a single layer in an 8-inch square baking dish. Pour brandy evenly over plums, and sprinkle with 1/4 cup sugar. Broil until plums are soft and sugar is caramelized, 8 to 10 minutes.
Meanwhile, put mascarpone, cream, and remaining 2 tablespoons sugar in the bowl of an electric mixer fitted with the whisk attachment; beat until soft peaks form, about 2 minutes. Top broiled plums with whipped mascarpone cream.