Preheat the oven to 160°C (320°F). Grease and line a cake pan with parchment paper.

In a saucepan, melt the dark chocolate and butter over low heat, stirring until smooth.

Remove from heat and stir in brown sugar and granulated sugar until well combined.

Add eggs one at a time, mixing well after each addition. Stir in vanilla extract.

Sift together flour, cocoa powder, baking powder, and salt. Gradually fold into the chocolate mixture until just combined.

Pour the batter into the prepared cake pan and smooth the top.

Bake for 60 minutes or until a skewer inserted into the center comes out with a few moist crumbs.

Allow the cake to cool in the pan for 10 minutes, then transfer to a wire rack to cool completely.

Melt the dark chocolate and butter together in a heatproof bowl over simmering water.

Remove from heat and whisk in powdered sugar and heavy cream until smooth and glossy.

Spread the frosting over the cooled cake and let it set before serving.