Preheat oven to 400 degrees and butter the pie plate well.
Sprinkle the pie plate with the 1 tablespoon sugar and distribute the plum halves over the sugar, skin side down. In a blender blend the milk, light cream, flour, eggs, egg yolk and salt for 2 minutes. Add the 1/4 cup sugar and the vanilla, blend the mixture for a few seconds and pour it over the plums. Bake the clafouti in the middle of the oven for about 30 minutes or until puffed and golden. Dust the clafouti with vanilla sugar and serve at once.