6 tablespoons unsalted butter, plus more for pan
8 ounces bittersweet or semisweet chocolate, finely chopped
6 large eggs, separating yolks and egg whites
1/2 cup granulated sugar
Confectioners’ sugar, for dusting
Sweetened whipped cream, for serving
Preheat the oven to 275 degrees with the rack in the center. Butter the bottom and sides of a 9-inch springform pan. Set aside.
Place butter and chocolate in a large heatproof bowl and microwave in 30-second increments, stirring each time, until completely melted. Let cool slightly. Whisk in egg yolks.
In a large bowl, beat egg whites until soft peaks form. Gradually add granulated sugar, and continue beating until glossy stiff peaks form. Whisk 1/4 of the egg whites into the chocolate mixture; then gently fold in remaining egg whites.
Pour batter into the prepared pan, and smooth the top with a rubber spatula. Bake until the cake pulls away from the sides of the pan and is set in the center, 45 to 50 minutes. Cool completely on a wire rack; remove sides of pan. Serve at room temperature, dusted with confectioners’ sugar. Serve with whipped cream, if desired.