Preheat the oven to 200C (400F). Butter a 6-cup muffin tin and dust with cocoa powder.
Melt the butter and chopped bittersweet chocolate together until smooth; let cool slightly.
Beat the eggs, egg yolks, sugar, and vanilla until pale and ribbon-like.
Fold the melted chocolate mixture into the egg mixture until combined.
Sift in the flour and fold gently until no streaks remain.
Spoon half the batter into the 6 prepared muffin cups.
Place one chilled hollow chocolate egg into each cup and cover with the remaining batter.
Bake for 10-12 minutes until the edges are set but the centers still wobble.
Cool for 2 minutes, then carefully invert onto plates and serve immediately.