Heat the oven to 350°F (180°C) then butter a cast 9 inch cast iron skillet, set aside. 

Cut the plums in half, working around the pit to remove it. Toss the plums with 1/3 cup of the sugar.

Once the plums have been coated in sugar, arrange them evenly in the skillet cut side up.

In a medium bowl, whisk the eggs, the remaining 1/3 cup sugar, flour, and salt. Lastly, add in the milk and vanilla and whisk until a batter is formed. 

Pour the batter over the plums into the cast iron pan.

Bake the clafoutis for about an hour or until it has risen, the center is set and the edges are golden brown. 

Remove from the oven and allow to cool for just 10 minutes. Dust with powders sugar and serve. Note: Clafoutis will deflate over time so best to serve it fresh from the oven.

Cover and store leftover clafoutis in an airtight container in the fridge for up to 24 hours.