Preheat oven to 425°F. Butter a 9-inch pie plate. Drain cherries; sprinkle with 1 Tbsp. sugar.
Whisk eggs and yolk together in a large bowl until blended. Add flour gradually, stirring constantly with a fork until incorporated. Stir in remaining 1/2 cup sugar, salt and vanilla extract. Slowly add milk, stirring, until mixture forms a thin batter.
Pour half of batter into bottom of pie pan. Bake until firm and set, about 10 minutes. Remove from oven and scatter cherries over batter. Pour remaining batter over cherries and return to oven. Cook until puffed, lightly browned and set in middle, about 25 minutes longer. Cool slightly on a wire rack. Sift confectioners' sugar over clafouti, if desired, and serve warm.