These small desserts, made in individual ramekins, are a cross between cake, pudding, and soufflé. They're meant to be eaten in the ramekins, rather than turned out. Feel free to substitute other berries, such as blueberries or blackberries, if you like. The cakes are best served warm, but can be made ahead and served at room temperature.
Position a rack in the lower third of the oven and preheat to 350 degrees F. Butter the top inch of the inside of each ramekin. Divide the raspberries among the ramekins, arranging them in a single layer on the bottom of the dishes. (If you have extra berries, they can be used for garnish.) Place the ramekins in a roasting pan or deep baking dish and set aside.
Sift together the flour, cornstarch, and salt. Set aside.
Place the egg yolks in a medium bowl and whisk in 3/4 cup of sugar. Add the flour mixture alternately with the milk in two stages, beginning with the flour. Whisk in the lemon juice, lemon zest, and the melted butter. Set the batter aside. (The cakes can be made up to 6 hours in advance to this point, then refrigerated; bring the batter to room temperature before proceeding.)
Bring a kettle or pan of water to a boil (this will be added to the roasting pan to make a water bath).
Meanwhile, beat the egg whites with an electric mixer until foamy. Then, with the machine running, gradually add the remaining 1/4 cup of sugar. Beat until the whites form stiff, shiny peaks, about 2 minutes on medium-high speed.
Whisk about half of the whites into the batter to lighten it. Then carefully fold in the remaining whites with a wide spoon or rubber spatula. Divide the batter among the ramekins. Transfer the roasting pan to the oven and carefully add enough boiling water so it comes halfway up the sides of the ramekins.
Bake until the cakes are set and the tops are lightly golden, 30 to 40 minutes. Using tongs, transfer each ramekin from the roasting pan to a wire rack. Let the cakes cool slightly before serving with a dollop of whipped cream and a sprig of mint.