[Preparation Steps]

Making the Brownie Sheet

1. Place a bowl over a pot of boiling water, add dark couverture chocolate and unsalted butter, and stir until melted and well combined.
2. Remove the bowl from the pot, then add sugar and salt and mix. Add the egg yolks and combine.
3. Sift in cake flour, cocoa powder, and baking powder, then fold gently.
4. Add warm milk (around 40°C) and mix, letting it sit for a moment.
5. In a separate bowl, add sugar to the egg whites in two portions and whip to form meringue.
6. Gently fold half of the meringue into the chocolate mixture, then transfer the mixture back into the meringue bowl and fold until well combined.
7. Pour the batter into an oven pan and bake at 165°C (330°F) for about 30 minutes, then let it cool.
8. Trim the raised edges and cut the sheet to fit a 15cm mold.

Making Pistachio Cheesecake

9. Mix gelatin powder with water and let it sit for about 10 minutes to bloom.

10. Soften room-temperature cream cheese, then add sugar and mix until smooth.

11. Add pistachio paste and mix well. Heat the bloomed gelatin in the microwave for about 15 seconds to melt, then add to the mixture.

12. Fold in half-whipped heavy cream (whipped to about 50%) and combine evenly.

13. Pour the mixture into a mold, smooth the surface, and freeze for about 20 minutes to set.

Making Ganache

14. Add heavy cream to dark chocolate and microwave for about 30 seconds to melt.

15. Add unsalted butter and stir until smooth.

16. Pour into the mold, smooth the surface, and freeze for about 2 hours to set.