Preheat oven to 350°F. Place butter in a 2–3-qt. shallow baking dish or medium (10") ovenproof skillet. Transfer to preheating oven and heat until butter has melted, about 4–6 minutes.
Meanwhile, blend eggs, milk, lemon zest, salt, vanilla, and 1/3 cup granulated sugar in a blender until frothy, about 2 minutes. Add flour and blend just until combined.
Pour batter into hot baking dish. Arrange fruit evenly over. Sprinkle with remaining 2 Tbsp. granulated sugar. Bake clafoutis until top is puffed and lightly golden brown, 25–35 minutes. Let cool slightly, then sift powdered sugar over (if using). Serve warm.
