1. Heat oven to 350 degrees F. Grease a 9-inch springform pan. Line the outside of the pan with foil.
2. Beat eggs and sugar in a large mixer bowl at medium-high speed 8 to 10 minutes, until thick. Reduce speed to medium; beat in liqueur and butter until blended. (Batter will look curdled, which is okay.) Reduce speed to low; beat in chocolate and vanilla until well combined. Pour batter into pan. Bake 30 minutes, just until set.
3. Let stand on a wire rack 30 minutes. Serve warm with whipped cream and raspberries, if desired. Makes 12 servings.