Preheat the oven to 375 degrees F. Butter a 9-inch pie plate or similar sized oven-safe dish, then sprinkle with a little bit of granulated sugar. Roll the dish around so the sugar sticks to the butter, then shake out any excess.

Add the milk, eggs, brown sugar, vanilla, salt and flour to a bowl or blender jar. Vigorously whisk or blend to combine (I often use an immersion blender). Scrape down the sides for any stuck bits of flour and blend until very smooth and no lumps remain.

Arrange the plum slices in the bottom of the prepared dish, then pour batter over top. Bake until puffed and lightly golden brown, about 30-40 minutes. Let cool at least 30 minutes before serving.

Serve warm, room temperature or ice cold (my preference).