Preheat your oven to 375°F (190°C). Generously butter a 9 or 10-inch cast-iron skillet or a shallow baking dish and dust lightly with a tablespoon of the granulated sugar mixture.

Arrange the stone fruit evenly over the bottom of the prepared dish. Sprinkle half of the remaining 1/3 cup of sugar and the seeds scraped from the vanilla bean (if using) over the fruit.

Prepare the batter: Whisk together the flour, 1/2 cup sugar, and salt in a large bowl. In a separate bowl, whisk the eggs together lightly. Gradually whisk the eggs into the dry ingredients until just combined; do not overmix.

Slowly whisk in the lukewarm milk and the Kirsch (if using) until the batter is smooth, thin, and has the consistency of heavy cream. If using vanilla extract, stir it in now.

Gently pour the batter evenly over the fruit in the prepared dish. The fruit should be mostly submerged or sitting just below the surface.

Bake for 40 to 45 minutes, or until the clafoutis is deeply golden brown, puffed around the edges, and a skewer inserted near the center comes out mostly clean (it should still be slightly moist, not dry).

Remove from the oven, let cool on a wire rack for at least 15 minutes before serving. Dust generously with powdered sugar before slicing.