Serves 8 to 10
FOR THE CAKE
1 stick (4 ounces/115 g) unsalted butter, cut into 8 pieces, plus extra for the pan
12 ounces (340 g) fine-quality bittersweet chocolate, chopped
1 teaspoon pure vanilla extract
¼ teaspoon kosher salt
⅔ cup (135 g) sugar, divided
5 large eggs, separated and at room temperature
1 tablespoon all-purpose flour
Confectioners powdered sugar, for dusting the cake
Lightly sweetened fresh whipped cream or crème fraîche, for serving
1 Blood orange, Cara Cara orange or Kumquats, for finishing