Heat the oven to 425°F
Butter an 8-inch oven-safe frying pan and set it aside.
Melt the chocolate, butter, and salt together in a heatproof bowl set over a pot of simmering water (bain-marie) or melt gently in the microwave in short bursts. Stir until smooth, then let it cool slightly.
Beat the eggs and sugar together in a stand mixer on high speed until the mixture is very light, voluminous, and falls off the whisk in thick ribbons. This usually takes at least 5 minutes.
Pour the melted chocolate mixture into the airy egg mixture. Mix on low speed just until combined. Stop the mixer and use a rubber spatula to scrape the bottom of the bowl, folding in any streaks of chocolate so the batter is evenly mixed without deflating too much.
Pour the batter into the prepared pan and smooth the top if needed.Bake for 9–10 minutes, or until the edges are puffed and set while the center still feels soft and slightly jiggly.
