Preheat oven to 425°F (220°C) and grease skull molds with non-stick spray or butter and cocoa powder.

Melt chocolate and butter together in a heatproof bowl over simmering water until smooth. Let cool slightly.

In a separate bowl, whisk eggs, yolks, sugar, and vanilla extract until thick and pale, about 2-3 minutes.

Gently fold the chocolate mixture into the egg mixture.

Sift in the flour and salt, folding just until combined.

Fill each mold 3/4 full with the batter.

Bake for 11-13 minutes until edges are set and centers are slightly jiggly.

Cool for 1-2 minutes, invert carefully onto plates, and serve immediately.