Notes: To have a molten center, this cake should be served immediately, while it's warm. It still has a delicious fudgy texture, though, if made (up to 1 day) ahead; cool cake completely, wrap loosely in plastic wrap, and store at room temperature. Reheat in a 300° oven for 10 minutes just before serving.
7 ounces bittersweet chocolate, finely chopped
2 cups plus 2 tablespoons granulated sugar
1 cup whipping cream
1/4 cup (1/8 lb.) butter
4 large eggs, separated
3 tablespoons all-purpose flour
How to Make It
In a bowl set over a pan of barely simmering water (bottom of bowl should not touch water), stir chocolate until melted and smooth. Remove pan from heat but leave bowl over water to keep chocolate warm.
In a 3- to 4-quart pan over medium heat, stir 2 cups granulated sugar in 1 cup water until sugar is dissolved. Increase heat to high and boil without stirring until mixture is a deep caramel color, 15 to 20 minutes. When sugar begins to brown around the edges of the pan, swirl pan gently to ensure the mixture caramelizes evenly. Remove from heat and immediately add cream and butter; whisk until smooth. Remove 1 cup caramel sauce and reserve.
Remove chocolate from over water and whisk in egg yolks, then whisk mixture into caramel in pan. Whisk in flour.
In a bowl, with a mixer on high speed, beat egg whites with remaining 2 tablespoons sugar just until soft peaks form. Fold a third of the egg whites into chocolate mixture, then gently fold in remaining whites until no streaks remain. Scrape mixture into a buttered and floured 8-inch cake pan.
Bake in a 325° regular or convection oven until a wooden skewer inserted 2 inches from the side of the pan comes out clean but center of cake is still wet, 20 to 25 minutes.
Cool cake in pan on a rack for 5 minutes, then run a knife between cake and pan rim. Invert cake onto a plate, then place another plate on top and invert to turn cake upright. Dust with powdered sugar and serve warm (see notes) with reserved caramel sauce.