Preheat your oven to 425°F (220°C) and liberally grease four 6-ounce ramekins with butter.

Dust the inside of each greased ramekin with cocoa powder, shaking out any excess to ensure the cakes release easily after baking.

In a heatproof bowl set over a pot of simmering water, melt the chopped chocolate and butter together, stirring until smooth.

In a separate medium bowl, whisk together the eggs, egg yolks, powdered sugar, and sea salt until the mixture is pale and thick.

Gently fold the melted chocolate mixture into the egg mixture until fully combined.

Sift the flour into the bowl and fold it in gently just until no white streaks remain, being careful not to overmix.

Pour the batter into the prepared ramekins, filling them about halfway up.

Spoon one tablespoon of salted caramel sauce into the center of each ramekin, then top with the remaining batter to cover the caramel completely.

Place the ramekins on a baking sheet and bake for 12-14 minutes, until the sides are firm but the centers still jiggle slightly when shaken.

Remove from the oven and let the cakes cool for 1 minute before running a small knife around the edges to loosen them.

Carefully invert each ramekin onto a dessert plate and lift off the mold to reveal the cake, serving immediately while warm.