How to Make It
Preheat oven to 350°F. Beat egg whites with an electric mixer on medium-high speed until stiff peaks form, about 3 minutes.
Place chocolate and butter in a 10-inch cast-iron skillet (or other oven-safe skillet) and cook over medium, stirring constantly, until melted, 2 to 3 minutes. Remove from heat and whisk in sugar, salt, and egg yolks. Gently fold in beaten egg whites until just combined (a few streaks are OK).
Transfer skillet to oven and bake until cake begins to pull away from sides of skillet and a toothpick inserted in center comes out mostly clean, about 25 minutes. Cool 10 minutes (center of cake will sink.) Serve warm with ice cream.