Preheat your oven to 350°F (175°C) and prepare a 9-inch round cake pan by greasing it lightly and lining with parchment paper.

In a medium heatproof bowl, melt dark chocolate and butter over simmering water, stirring until smooth for about 5 minutes.

Whisk together sugar and eggs in a large bowl until pale and fluffy (about 4-5 minutes), then gradually mix in cooled chocolate.

Stir creamy peanut butter into the chocolate-egg mixture until fully combined and smooth.

Pour batter into the cake pan and bake for 30-35 minutes until a toothpick inserted comes out clean.

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

Whip heavy cream, sugar, and melted chocolate in a clean bowl until soft peaks form.

Spread mousse evenly over the cooled cake and refrigerate for at least 2 hours to set.

Slice and serve chilled, optionally garnished with chocolate shavings or fresh berries.