Preheat oven to 350°F (175°C) and grease two 9-inch round cake pans.

Melt dark chocolate and butter in a double boiler, then set aside to cool.

Whisk eggs and sugar in a bowl until fluffy, then fold in the melted chocolate mixture.

Sift cocoa powder and flour into the batter, folding gently to combine.

Pour batter into prepared pans and bake for 25-30 minutes until a toothpick inserted comes out clean. Cool completely.

Whip heavy cream until soft peaks form; mix sugar with chopped mint leaves and combine with whipped cream.

Assemble by layering one cake on a plate, spreading half the mint mousse on top, adding the second layer, and topping with remaining mousse.