To make the custard, in a saucepan over medium heat, warm the milk, sugar, and vanilla bean, stirring constantly to dissolve the sugar. In a small bowl, whisk the egg yolks to break them up, but don't make them foam. Whisk a little of the hot milk into the egg yolks to warm them. Return the eggs to the pan and cook the custard, stirring constantly, until it coats the back of a spoon and reaches approximately 170°F. Test it by drawing your finger across the back of the spoon: if your finger leaves a trail in the custard, the custard has cooked to the right point. Immediately strain into a bowl and chill.
To make the cakes, preheat the oven to 400°F. Butter and flour 6 ramekins, 5 to 6 ounces each.
In the top of a double boiler over medium heat, heat the chocolate, butter, and Cognac, stirring until smooth and the chocolate and butter are melted. Set aside.
In a bowl, beat the granulated sugar and the 4 egg yolks until ribboned and light colored, 1 to 2 minutes. In another bowl, beat the 4 egg whites until stiff. Gently fold the egg whites into the egg yolk mixture. Sift the flour over the top and fold the flour and chocolate mixture into the egg mixture. Evenly distribute the mixture among the prepared ramekins. Place the ramekins on a baking sheet and bake until the cakes have risen and the tops just begin to crack, 10 to 12 minutes.
Immediately remove the ramekins from the oven and run a knife around the edges of the ramekins. Gently invert the ramekins onto 6 dessert plates. Spoon the custard sauce around the edges and dust the tops with confectioners' sugar. Garnish with raspberries and serve warm.