Preheat the oven to 350°F (175˚C). Grease a 9-inch (23-cm) springform pan with butter, fit the bottom with a round of parchment paper, and grease the paper.
Combine the chocolate and butter in a large metal bowl, set it over a saucepan of simmering water, and stir frequently until the chocolate and butter are completely melted. Remove from the heat and let the mixture cool completely. Whisk in the vanilla extract, salt, and ⅓ cup (65 g) of the sugar. Add the egg yolks one at a time, whisking well after each addition. Whisk in the flour.
In a medium bowl, use an electric mixer set at medium-high speed to beat the egg whites together with a pinch of salt. Beat until soft peaks form, then add the remaining ⅓ cup (65 g) sugar a little at a time, and continue beating the whites until they hold stiff, glossy peaks.
Whisk about a quarter of the egg whites into the chocolate mixture, then use a rubber spatula to gently fold in the remainder. Pour the batter into the springform pan, spreading it evenly across the pan. Bake for 28 to 30 minutes, or until the cake’s center is slightly gooey and its edges pull away from the pan.
When the cake is finished baking, transfer it to a wire rack and let it cool for 10 minutes, then gently unmold it from the pan and let it cool to room temperature. Adorn the cake with sugar coated kumquats or citrus slices. Dust the top with powdered sugar.
Serve each piece of cake piled with the whipped cream or crème fraîche
