Heat oven to 350 degrees.
Lightly butter a medium-size flameproof baking dish at least 1 1/2 inches deep.
Place the milk, 1/3 cup granulated sugar, eggs, vanilla, salt and flour in a blender. Blend at top speed until smooth and frothy, about 1 minute.
Pour a 1/4-inch layer of batter in the baking dish. Turn on a stove burner to low and set dish on top for a minute or two, until a film of batter has set in the bottom of the dish. Remove from heat.
Spread berries over the batter and sprinkle on the remaining 1/3 cup granulated sugar.
Pour on the rest of the batter and smooth with the back of a spoon.
Place in the center of the oven and bake about 50 minutes, until top is puffed and browned and a tester plunged into its center comes out clean.
Sprinkle with powdered sugar just before serving. (Clafoutis need not be served hot, but should still be warm. It will sink slightly as it cools.)