in a small saucepan, whisk 4 tablespoons granulated sugar with the 4 egg yolks and then stir in the Armagnac and salt. 

Place over medium heat, and cook, stirring constantly, until the custard is thickened, about 8 minutes. Remove the custard from the heat and stir in the vanilla.

In a large bowl, whisk the 2 egg whites with the remaining 2 tablespoons granulated sugar until they hold soft peaks. Scrape the whites into the custard and fold until evenly combined.

Heat the broiler. In an 8-inch round baking dish, arrange the plums cut side up and broil until heated through and beginning to brown, about 2 to 3 minutes. 

Remove the dish from the broiler, spoon the custard over and around the plum halves, and return to the broiler until the custard turns golden brown, about 2 minutes.

Dust the plums and custard with confectioners’ sugar and serve hot right from the dish.