Preheat your oven to 350F. Grease a big baking dish with 3 tablespoons of butter, then spread out the cherries and blueberries. Set it aside.
In a bowl, whisk the eggs with half a cup of sugar. Mix in the flour thoroughly. Add the melted butter, milk, and vanilla extract, and stir until everything's well combined.
Pour the batter over the cherries and blueberries in your baking dish.
Bake the clafoutis for about 35-40 minutes, or until a skewer inserted comes out clean.
Take the dish out of the oven and sprinkle with the remaining superfine sugar and powdered sugar.