Butter a medium baking dish, ~2-quart and at least 1 1/2-inches deep. (see recipe note #2)
Combine milk, 1/3 cup of the sugar, eggs, vanilla, salt and flour in a blender pitcher; blend until smooth and foamy, about 1 minute. Pour 1/4-inch layer in to prepared baking dish and place in oven for 5 minutes, or until batter has set. Remove from oven.
Spread peach slices evenly over set batter. Sprinkle remaining sugar (~2 tablespoons) over peach slices. Pour on remaining batter. Bake in the center of the oven for 50-55 minutes, until the clafoutis is puffed and golden brown and a sharp knife or cake tester inserted into the center comes out clean. Remove from oven and let cool slightly.
Just before serving, spoon powdered sugar into a fine sieve or shaker and sprinkle over the clafoutis. Serve warm.
Notes
To peel the peaches easily, bring a pot of water to a boil and score an "X" on the bottom of each peach. Blanch the peaches for 30 seconds, then immediately transfer them to an ice bath. Once they're cool, the skins should slide right off.