1. Prepare the pan.
Preheat the oven to 350°F (175°C). Grease a 9-inch round cake pan. Line the bottom with parchment paper.

2. Mix the dry ingredients.
Whisk flour, baking powder, and salt together in a medium bowl.

3. Cream butter and sugar.
Beat softened butter and granulated sugar for about 3 minutes, until pale and fluffy.

4. Add the eggs.
Beat in the eggs one at a time. Mix well after each addition. Add vanilla.

5. Finish the batter.
Add the flour mixture in three additions, alternating with the milk. Begin and end with flour. Mix only until combined.

6. Bake.
Transfer the batter to the prepared pan and smooth the top. Bake for approximately 25–30 minutes, or until a toothpick inserted near the center comes out clean or with a few moist crumbs.

7. Cool completely.
Cool in the pan for 10 minutes. Turn onto a wire rack and allow the cake to cool completely before cutting.

8. Make the blackberry purée.
Blend blackberries, granulated sugar, and lemon juice until smooth. Push the mixture through a fine-mesh sieve. Discard the seeds.

9. Bloom the gelatin.
Place 3 tablespoons cold water in a small bowl. Sprinkle the gelatin evenly over the water. Leave for 5 minutes.

10. Dissolve the gelatin.
Warm about ¼ cup of blackberry purée over low heat. Do not boil. Remove from the heat and stir in the bloomed gelatin until completely dissolved.

11. Finish the blackberry mixture.
Stir the gelatin mixture into the remaining blackberry purée. Allow it to cool to room temperature. It should remain fluid.

12. Prepare the cream cheese mixture.
Beat softened cream cheese and powdered sugar until completely smooth. Add vanilla, followed gradually by the cooled blackberry mixture.

13. Whip the cream.
Whip the cold heavy cream to soft-to-medium peaks.

14. Make the mousse.
Fold one-third of the whipped cream into the blackberry mixture. Fold in the remaining cream in two additions. Do not beat or aggressively stir.

15. Divide the cake.
Once completely cold, level the cake if necessary. Carefully cut it horizontally into three thin layers.

16. Assemble.
Line a 9-inch cake ring or springform pan with food-safe acetate. Place one vanilla cake layer in the bottom. Add half the blackberry mousse and level it.

Add the second cake layer. Spread over the remaining mousse. Finish with the third vanilla cake layer.

17. Chill.
Cover and refrigerate for at least 6 hours, preferably overnight.

18. Make the topping.
Place finely chopped chocolate in a heatproof bowl. Heat ½ cup heavy cream until steaming but not boiling. Pour it over the chocolate.

Wait 2 minutes. Stir slowly until smooth. Mix in blackberry preserves and butter.

19. Cool the ganache.
Leave it for approximately 10–15 minutes. It should be glossy, slightly thickened, and pourable rather than hot.

20. Finish the cake.
Remove the chilled cake from its ring. Remove the acetate. Spoon the blackberry chocolate ganache over the top, allowing some to drip naturally over the edge.

21. Serve.
For neat slices, warm a sharp knife under hot water, wipe it completely dry, and cut. Clean and dry the knife between each slice.

Serve chilled.