1. Preheat oven to 350°F (175°C). Grease and line a 9-inch springform pan.

2. Whisk flour, cocoa powder, baking powder, baking soda, and salt in a bowl.

3. Cream butter and sugar until light and fluffy (2–3 minutes).

4. Add eggs one at a time, mixing after each. Mix in vanilla.

5. Mix in buttermilk, then add dry ingredients and mix just until combined.

6. Pour batter into pan and bake 25–35 minutes, until a toothpick comes out clean. Cool completely.

7. Melt dark chocolate, milk chocolate, and white chocolate separately. Let each cool slightly.

8. Whip heavy cream to soft peaks and divide evenly into 3 bowls.

9. Fold melted dark chocolate into the first bowl of whipped cream until smooth.

10. Fold melted milk chocolate into the second bowl of whipped cream until smooth.

11. Fold melted white chocolate into the third bowl of whipped cream until smooth.

12. (If using egg whites for extra lift) Whip egg whites to soft peaks and gently fold equal portions into each mousse.

13. Place cooled cake back into springform pan. Spread dark chocolate mousse evenly over cake. Chill 15–20 minutes.

14. Spread milk chocolate mousse over the dark layer. Chill 15–20 minutes.

15. Spread white chocolate mousse on top. Cover and chill at least 6 hours or overnight until fully set.

16. Slice with a warm, clean knife for sharp layers. Serve chilled.