Preheat your oven to 350°F (175°C). Grease and line a cake pan.
Melt the dark chocolate and butter together in a heatproof bowl over simmering water. Let it cool slightly.
In a separate bowl, whisk the egg yolks with half of the sugar until pale and thick. Fold in the melted chocolate mixture.
In another bowl, whisk the egg whites until soft peaks form. Gradually add the remaining sugar and whisk until stiff peaks form.
Gently fold the egg whites into the chocolate mixture, followed by the flour. Pour the batter into the prepared cake pan and bake for 25-30 minutes. Let it cool completely.
For the mousse, melt the dark chocolate and let it cool slightly. Whip the heavy cream with the sugar until soft peaks form. Fold in the melted chocolate.
Spread the mousse over the cooled cake base. Refrigerate for at least 2 hours before serving.