1. Preheat the oven to 350°F (180°C). Line the bottom of 9-inch springform pan with waxed or parchment paper. Don’t grease the pan —the batter needs to cling to the sides as it rises.

Cake

1. Gently melt the chocolate with the butter over low on the stovetop, or in a glass or stainless steel bowl set over a pot of simmering water. Remove from the heat and set aside to cool slightly.
2. Separate 4 of the eggs, putting the yolks and whites in separate medium bowls. Add the remaining 2 eggs and 1/2 cup of the sugar to the egg yolks. Whisk in the warm chocolate mixture and stir until smooth.
3. Beat the egg whites with an electric mixer until foamy.
4. Gradually add the remaining sugar, beating until the egg whites form soft mounds but aren’t yet stiff. Fold about one-quarter of the egg whites into the chocolate mixture to lighten it, and then gently fold in the rest, without deflating the egg whites. Pour the batter into the prepared pan and smooth the top.
5. Bake for 35 to 40 minutes, until the cake is puffy and cracked on top, and the middle isn’t wobbly. Cool the cake completely in the pan without loosening the sides (the batter will cling to the pan is it cools, sinking in the middle and keeping its high sides).

Raspberry Fool

1. Meanwhile, in a medium bowl, gently crush the berries with half the sugar —a potato masher or whisk works well. Set aside to macerate for 10 minutes or so.
2. When you’re ready to serve the cake, whip the cream with the rest of the sugar until softly stiff, and fold in the fruit mixture, leaving it swirled through the cream rather than blended in.
3. Run a knife around the edge of the pan to loosen the sunken cake and remove the sides of the pan. Transfer the key to a serving plate, leaving the cake on the pan bottom. Mound the fool in the middle of the cake. Serve immediately.