Preheat oven to 375 degrees. Grease a 13-inch by 9-inch baking pan. In a sauce pan, add the raspberries and blueberries. Using a fork, slightly macerate the berries. To the berries, add 1 cup sugar and the Chambord. Bring the mixture to a boil and reduce to a simmer. Whisk in the cornstarch slurry and continue to cook for about 3 to 4 minutes, just until the berries thicken. Remove from the heat and pour into the prepared pan. Using an electric mixer, whip the eggs until light in color and have a ribbon-like texture, about 3 minutes. With the mixer running, slowly add the sugar. Combine the milk and butter together. Stir the milk mixture into the whipped yolks. Fold the flour into the egg mixture. Pour the batter evenly over the berries. Run a knife through the batter to bring some of the liquid from the berries to the surface. Bake for 30 to 35 minutes, or until the batter is set and golden brown. Remove the cake from the oven and allow to cool. Spoon some of the cake into a shallow bowl, serve with the ice cream. Garnish with the fresh mint sprigs and powdered sugar.