1Melt the chocolate and butter over a double boiler until melted. Allow to cool.
2Separate 4 eggs. Whisk 2 whole eggs and 4 egg yolks with 100g of sugar until combined.
3Slowly whisk in the warm chocolate mixture.
4Beat the 4 egg whites with a handheld mixer until foamy (about 2 minutes).
5Gradually add the remaining 100g of sugar and beat until soft peaks form (about 5 more minutes).
6Gently fold about a quarter of the beaten egg whites into the chocolate mixture to lighten it, then gently fold in the remaining whites.
7Scrape the batter into the pan and smooth the top. Set the pan on a rimmed baking sheet and bake at 175C until the top is puffed and cracked and the center is no longer wobbly (35 to 40 minutes).
8Let the cake cool in the pan on a rack. The center will sink as it cools.