Follow these easy steps to create your Pistachio Mousse with Chocolate Ganache:

For the Pistachio Mousse:

1. Prepare Pistachios: Place the shelled pistachios in a food processor and pulse until finely ground.
2. Whip Cream: In a mixing bowl, whip the heavy cream until soft peaks form. Set aside.
3. Dissolve Gelatin: In a small bowl, combine the gelatin and water. Let it sit for about 5 minutes until it softens, then heat it gently until dissolved.
4. Mix Ingredients: In another bowl, combine the ground pistachios, sugar, vanilla extract, and salt. Stir in the dissolved gelatin.
5. Fold in Cream: Gently fold the whipped cream into the pistachio mixture, being careful not to deflate the cream.
6. Chill Mousse: Spoon the mousse into serving glasses or bowls and refrigerate for at least 1 hour to set.

For the Chocolate Ganache:

1. Heat Cream: In a small saucepan, heat the heavy cream over medium heat until it just begins to simmer. Do not boil.
2. Melt Chocolate: Place the chopped chocolate in a heatproof bowl. Pour the hot cream over the chocolate and let it sit for 5 minutes.
3. Stir Ganache: Stir the mixture until smooth and fully combined. If desired, add the butter for extra shine.
4. Cool Ganache: Allow the ganache to cool slightly before using.

Assembling the Dessert:

1. Layer Ganache: Once the mousse has set, gently pour the chocolate ganache on top of each mousse layer.
2. Chill Again: Return to the refrigerator for another 30 minutes to allow the ganache to set.
3. Serve: Garnish with crushed pistachios or chocolate shavings if desired, and serve chilled.