Creating this Pistachio Mousse with Chocolate Ganache is a rewarding experience when you follow these straightforward steps:

1. Prepare the Pistachios: Start by grinding the pistachios in a food processor until they resemble a fine powder. Set aside.

2. Whip Cream: In a mixing bowl, whip 1 cup of heavy cream until stiff peaks form. Set aside in the refrigerator.

3. Make Pistachio Mixture: In another bowl, combine the ground pistachios, granulated sugar, and vanilla extract. Mix well.

4. Dissolve Gelatin: In a small bowl, sprinkle gelatin over water and let it sit for about 5 minutes to bloom. Then, heat gently until dissolved.

5. Combine Mixtures: Add the dissolved gelatin to the pistachio mixture and stir until well combined.

6. Fold in Whipped Cream: Gently fold the whipped cream into the pistachio mixture until fully incorporated. Be careful not to deflate the whipped cream.

7. Chill Mousse: Pour the pistachio mousse into serving glasses or a large dish. Cover and chill in the refrigerator for at least 1 hour.

8. Prepare Ganache: In a saucepan, heat 1 cup of heavy cream until simmering. Remove from heat and add chocolate chips and butter. Stir until smooth.

9. Cool Ganache: Allow the ganache to cool slightly, thickening as it does.

10. Assemble: Once the mousse has set, pour the ganache over the top of the mousse layer. Chill for an additional 30 minutes to set the ganache.

11. Garnish: Optionally, sprinkle extra chopped pistachios on top for added texture and presentation.