How to Make It
In a bowl, toss the cherries with 3 tablespoons of the granulated sugar and the lemon zest. Spread the cherries out on a baking sheet and freeze for 1 1/2 hours.
Meanwhile, in another bowl, whisk the 1/2 cup flour and salt. Whisk in the eggs. In a small saucepan, heat 1/2 cup of the milk with 3 tablespoons of the butter until the butter melts. Whisk the warm milk into the flour mixture just until smooth. Whisk in the remaining 1 1/2 cups of milk. Add the Cognac and vanilla, cover and let rest at room temperature for at least 1 hour.
Preheat the oven to 425°. Butter a 9 1/2-inch deep-dish pie plate and dust with flour. Spread the cherries in a single layer in the pie plate, adding any sugar from the baking sheet to the cherries. Whisk the batter again and pour it over the cherries.
Bake the clafoutis just above the center of the oven for 20 minutes, or until the top is just set and golden. Top with the remaining 1 tablespoon of granulated sugar and 1 tablespoon of butter. Bake for 10 to 15 minutes longer, or until a knife inserted in the center comes out clean. Transfer to a rack to cool. Dust with confectioners' sugar, cut into wedges and serve.