1. Preheat oven to 350°F and grease a round cake pan. Line the bottom with parchment paper.

2. In a bowl, whisk together flour, cocoa powder, baking powder, and salt.

3. In another bowl, cream butter and sugar until fluffy. Add eggs one at a time, then vanilla extract.

4. Gradually mix in the dry ingredients until just combined.

5. Pour the batter into the pan and bake for 20 minutes or until a toothpick comes out clean. Let cool completely.

6. Melt dark chocolate gently over a double boiler and let cool slightly.

7. In another bowl, whip heavy cream to soft peaks and refrigerate.

8. Over a double boiler, whisk egg yolks with sugar until thickened and pale. Add peppermint extract.

9. Fold the melted chocolate into the egg yolk mixture. Then gently fold in the whipped cream.

10. (Optional) Stir in melted gelatin for added mousse stability.

11. Place the cooled cake base into a springform pan. Pour mousse over the cake and smooth the top.

12. Refrigerate for at least 4 hours or overnight until fully set.

13. Unmold the cake and garnish with chocolate shavings, mint leaves, or ganache before serving.