How to Make It
Preheat the oven to 350°F. Butter and line an 8-inch round cake pan or springform pan with parchment. Butter the parchment and dust lightly with flour.
Melt the butter and chocolate in the microwave in 1-minute increments, on half power, stirring every minute, until shiny and smooth. Stir in the salt and set aside.
Whisk together the egg yolks and 4 tablespoons of the sugar until thick and creamy.
Mix the egg whites in a separate, clean bowl with an electric mixer on high until foamy. Gradually add the remaining sugar, a tablespoon at a time, whipping continuously until the whites are thick and glossy and hold a soft peak, 3 to 5 minutes.
Fold the egg yolk mixture into the chocolate. Add ⅓ of the chocolate mixture to the egg whites and fold in gently, until marbled. Taking care to keep the batter light and airy, fold in the flour in 2 parts, alternating with the remaining chocolate, until evenly combined.
Spoon the batter into the pan and bake until crispy on top and a toothpick inserted in the center comes out almost clean, 25 to 30 minutes. Cut into wedges and serve slightly warm with whipped cream.