1. Preheat the oven to 350°F and prepare a springform pan by greasing it and wrapping the outside with foil to prevent water from seeping in.
2. Melt the bittersweet chocolate and unsalted butter together in a heatproof bowl over a pan of simmering water, stirring until smooth.
3. Remove the melted chocolate mixture from the heat and let it cool slightly.
4. Separate the eggs, placing the yolks in one bowl and the whites in another.
5. Whisk the egg yolks with sugar until pale and thick, then stir in the brandy and the cooled chocolate mixture.
6. In a clean bowl, whip the egg whites until soft peaks form.
7. Gently fold the whipped egg whites into the chocolate mixture in batches, taking care to maintain the airy texture.
8. Pour the batter into the prepared springform pan.
9. Place the pan inside a larger baking dish and pour in hot water to create a water bath.
10. Bake for 45 minutes or until the cake is set but still slightly soft in the center.
11. Remove the pan from the water bath and let the cake cool completely in the pan.
12. As it cools, the center will sink, creating a crater-like appearance.
13. Once fully cooled, remove the cake from the springform pan and transfer it to a serving plate.
14. Fill the sunken center with whipped cream or sour cream for a creamy contrast.
15. Garnish with fresh raspberries or serve plain for an intense chocolate experience.
16. Slice and serve at room temperature to allow the flavors to fully develop.