Preheat oven to 350°F (175°C), with a rack in the center.
Line the bottom of an 8-inch (20cm) springform pan with parchment paper. (Do NOT butter the pan and parchment.)
Melt the chocolate in a heatproof bowl set over but not touching gently simmering water in a saucepan. You can whisk it occasionally to help it along. When it’s melted, remove the bowl from the heat and whisk in the butter until smooth.
In two small bowls, separate 4 of the eggs. In a large bowl, whisk 2 whole eggs and the 4 egg yolks with 1/2 cup (100g) of the sugar just until combined.
Slowly whisk in the warm chocolate mixture.
Whisk in the liqueur.
Using a handheld mixer in a separate bowl, beat the 4 egg whites until foamy, about 2 minutes. Gradually add the remaining 1/2 cup (100g) sugar and beat until beautifully glossy, soft peaks form that hold their shape but aren’t quite stiff, about 5 minutes more.
Very gently fold about a quarter of the beaten egg whites into the chocolate mixture to lighten it, then gently fold in the remaining whites. Scrape the batter into the pan and smooth the top.
Set the pan on a rimmed baking sheet and bake until the top is puffed and cracked and the center is no longer wobbly, 35 to 40 minutes. Be careful not to bake the cake beyond this point.
Let the cake cool completely on a rack. The center of the cake will sink as it cools, forming a sort of crater—this is good!
Fill crater with whipped cream.