1Preheat the oven to 350°F. Line the bottom of a 9" springform pan with a circle of waxed paper or parchment – use the bottom of the pan to trace a circle on the paper, then cut it out with scissors. Don’t grease the pan – the batter needs to be able to cling to the sides as it rises.
2Gently melt the chocolate with the butter in a glass or stainless steel bowl set over a bowl or pot of hot or gently simmering water, or do it on low in the microwave. (If you have a double boiler, use it, but it’s not essential.) The thing to remember when melting chocolate is to do it gently, and not let it come in contact with intense heat; chocolate scorches and can seize up easily.
3Separate 4 of the eggs, putting the yolks and whites in separate medium-sized bowls. Add the remaining 2 eggs and half of the sugar to the egg yolks. Whisk in the warm chocolate mixture and stir until it’s smooth.
4Beat the egg whites with an electric mixer until foamy. 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.
5Bake for 35-40 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 needs to cling to the sides of the pan as it cools so that it can properly sink in the middle and keep its high edges.
6If you're making fool for on top, gently crush the berries with half the sugar - I did this with a potato masher. Set aside to macerate for 10 minutes or so. When you’re ready to serve the cake, beat the cream with the rest of the sugar (if you are making fool - otherwise just with 2-3 Tbsp. sugar) until softly stiff and mound the whipped cream in the middle of the cake. Run a thin knife around the edge of the pan to loosen the cake, remove the sides of the pan and transfer it to a serving plate, leaving the cake on the pan bottom.