Preheat oven to 375°F (190°C). Grease an 8-inch round cake pan or line it with parchment paper.

Melt dark chocolate and butter in a medium saucepan over low heat until smooth. Let cool slightly.

In a large bowl, whisk together sugar and eggs until light and fluffy (about 3-5 minutes).

Gradually mix the melted chocolate mixture into the egg-sugar mix while stirring continuously. Add cocoa powder and vanilla; mix until well combined.

Pour batter into the prepared pan and spread evenly with a spatula.

Bake for about 25 minutes or until edges are set but center jiggles slightly. Allow cooling before serving.