Preheat the oven to 400°F (204°C). Generously butter an 8-inch oven-safe skillet, making sure the bottom and sides are evenly coated. Set aside.
Place the chopped chocolate and butter into a microwave-safe bowl. Microwave in 20-second intervals, stirring after each interval until completely melted and smooth.
Alternatively, melt using a double boiler. Allow the mixture to cool for about 5 minutes.
In a large mixing bowl, whisk together granulated sugar, egg yolks, and vanilla extract.
Whisk for about one minute until the mixture becomes lighter in color.
Slowly pour the cooled chocolate mixture into the egg mixture while whisking continuously. Mix until completely smooth.
Add flour, cocoa powder, espresso powder, and sea salt. Using a rubber spatula, gently fold everything together until no dry flour remains. Do not overmix.
Pour the batter into the prepared skillet. Spread evenly using an offset spatula or the back of a spoon. Lightly tap the skillet on the countertop once or twice to remove large air bubbles.
Bake for 17–21 minutes. The edges should look fully baked while the center still has a gentle jiggle.
The middle should remain glossy. Do not overbake.
Remove from the oven. Allow the skillet to rest for exactly 5 minutes. This finishes cooking the edges while keeping the center molten.
Top with vanilla ice cream, berries, chocolate curls, or whipped cream.
Serve immediately while the center is warm and flowing.