Preheat your oven to 350°F (180°C) and grease a baking dish or pie dish.
Arrange the sliced plums in a single layer in the bottom of the dish.
In a mixing bowl, whisk together the gluten-free flour, sugar, and salt.
In a separate bowl, beat the eggs, then gradually whisk in the milk and vanilla extract.
Slowly add the wet ingredients to the dry ingredients, stirring until the batter is smooth.
Pour the batter over the plums in the baking dish.
Bake for about 30-35 minutes, or until the clafoutis is set and golden brown on top.
Allow the clafoutis to cool slightly, then dust with powdered sugar before serving.