Place dark chocolate, butter, and cocoa powder in a bowl. Heat over a hot water bath until melted.

In a separate bowl, whisk eggs and sugar until the sugar dissolves completely.

Add the melted chocolate mixture to the egg mixture.

Sift the cake flour into the mixture and fold gently using a rubber spatula.

Pour the batter into a round baking tin lined with parchment paper. Tap the tin on the counter to release air bubbles.

Sprinkle nuts on top for garnish.