Preheat oven to 425°F. Grease four 6-oz ramekins generously with butter and dust lightly with cocoa powder, tapping out the excess.
Place the sugar-free dark chocolate and butter in a microwave-safe bowl. Microwave in 30-second intervals, stirring between each, until completely smooth and melted. Let cool for 2 minutes.
In a separate bowl, whisk together the eggs, egg yolks, powdered erythritol, and vanilla extract until pale and slightly thickened, about 2 minutes of vigorous whisking.
Pour the melted chocolate mixture into the egg mixture and fold gently until just combined. Sift in the cocoa powder and sea salt, then fold again until smooth. Do not overmix.
Divide the batter evenly among the prepared ramekins. Place them on a baking sheet and bake for exactly 12 to 13 minutes, until the edges are firm but the center still jiggles slightly when gently shaken.
Let the cakes rest for 1 minute, then run a thin spatula around the edges. Invert each ramekin onto a plate, wait 10 seconds, and carefully lift the ramekin off. Serve immediately while the centers are still molten.
