Preheat the oven to 180°C (350°F). Grease a 23 cm (9-inch) round baking dish with butter.
In a large bowl, whisk together 100 grams of all-purpose flour, 100 grams of granulated sugar, and a pinch of salt.
In another bowl, whisk together 3 large eggs, 250 ml of whole milk, 1 teaspoon of vanilla extract, and the zest of 1 lemon. Gradually add the wet ingredients to the dry ingredients, whisking until a smooth batter is formed.
Rinse and dry 500 grams of seedless grapes. Scatter the grapes evenly across the bottom of the prepared baking dish.
Pour the batter over the grapes, ensuring they are evenly covered.
Bake the clafoutis in the preheated oven for 40-45 minutes, or until the top is golden brown and a toothpick inserted into the center comes out clean.
Allow the clafoutis to cool slightly before dusting with powdered sugar and serving warm.