1. Preheat the oven to 325 degrees F. Butter an 8-inch springform pan and set it on a baking sheet.
2. In a bowl set over a saucepan with 1 inch of simmering water, combine the butter with the semisweet and unsweetened chocolate. Stir until the chocolate is melted, about 6 minutes; let cool.
3. In a medium bowl, beat the egg whites until stiff peaks form. In another bowl, beat 1 cup of the granulated sugar with egg yolks, then stir in half of the egg whites. Using a spatula, fold in the remaining egg whites.
4. Pour the batter into the prepared pan and bake for 50 minutes, or until the cake is well risen and cracked in a few places. Transfer to a rack and let stand for 10 minutes. Run a knife between the side of the pan and the cake, them remove the side and let the cake cool completely.
5. In a food processor, puree 1 pint of the raspberries with the remaining 2½ tablespoons of granulated sugar and the liqueur; strain to remove the seeds.
6. Cut a heart-shaped template from a piece of stiff paper large enough to cover the cake. Place the outside of the template on the cake and sift the confectioners' sugar over the top; carefully remove the template. Run a knife under the cake and transfer it to a cake plate. Alternatively, cut the cake into wedges and serve on individual plates. Garnish with the mint, the remaining 1/2 pint of raspberries and the raspberry sauce. Pass the additional raspberry sauce on the side.
Make Ahead: The cake and sauce can be refrigerated for up to 1 day. Serve cold or at room temperature.
Wine: 1988 Chateay Rieussec Sauternes