Preparation Put oven rack in upper third of oven and preheat oven to 400°F. If using Cognac, toss grapes with it in a small bowl. Pulse half of almonds with 1 tablespoon sugar in a blender until finely ground. Transfer to a small bowl and repeat with remaining almonds and 1 more tablespoon sugar. Beat together butter and remaining 6 tablespoons sugar in a bowl with an electric mixer at high speed until pale and fluffy, then beat in vanilla. Add eggs 1 at a time, beating well after each addition. Reduce speed to low and beat in ground almonds, flour, and salt until just combined. Divide batter among gratin dishes and press grapes lightly into batter (discard Cognac). Bake in dishes on a baking sheet until cake is firm and pale golden with slightly darker edges, about 20 minutes. Transfer cakes to a rack and cool slightly in dishes before serving.
Preparation Put oven rack in upper third of oven and preheat oven to 400°F. If using Cognac, toss grapes with it in a small bowl. Pulse half of almonds with 1 tablespoon sugar in a blender until finely ground. Transfer to a small bowl and repeat with remaining almonds and 1 more tablespoon sugar. Beat together butter and remaining 6 tablespoons sugar in a bowl with an electric mixer at high speed until pale and fluffy, then beat in vanilla. Add eggs 1 at a time, beating well after each addition. Reduce speed to low and beat in ground almonds, flour, and salt until just combined. Divide batter among gratin dishes and press grapes lightly into batter (discard Cognac). Bake in dishes on a baking sheet until cake is firm and pale golden with slightly darker edges, about 20 minutes. Transfer cakes to a rack and cool slightly in dishes before serving.