Preheat oven to 350°F (175°C). Grease and line an 8-inch round baking pan with parchment paper.

Melt the dark chocolate and butter together in a microwave-safe bowl, stirring frequently until smooth.

In another bowl, whisk eggs and sugar until pale and fluffy (about 5 minutes).

Gently fold the melted chocolate into the egg mixture using a spatula.

Stir in vanilla extract. In a separate bowl, whip heavy cream until soft peaks form, then fold it into the chocolate mixture until just combined.

Pour batter into the prepared pan and bake for 25-30 minutes until set but slightly jiggly in the center. Cool completely before removing from the pan.