Making Pistachio Mousse with Chocolate Ganache is simple when you follow these steps:

1. Prepare the Pistachios: Place the shelled pistachios in a food processor and pulse until finely ground.
2. Heat the Cream: In a small saucepan, heat 1 cup of heavy cream over medium heat until it begins to simmer. Remove from heat.
3. Dissolve Gelatin: In a small bowl, combine the gelatin and water. Allow it to sit for a few minutes until it thickens, then add it to the warm cream, stirring until dissolved.
4. Whisk Egg Whites: In a separate bowl, beat the egg whites until soft peaks form. Gradually add the granulated sugar while continuing to beat until stiff peaks form.
5. Combine Mixtures: Gently fold the ground pistachios and vanilla extract into the cream mixture. Then carefully fold in the whipped egg whites, being careful not to deflate them.
6. Chill the Mousse: Pour the mousse mixture into serving cups or a large dish. Refrigerate for at least 1 hour to set.
7. Prepare the Ganache: In a saucepan, heat ½ cup of heavy cream until it simmers. Remove from heat and add the semi-sweet chocolate chips. Stir until smooth and glossy.
8. Add Cocoa Powder: Sift in the cocoa powder and a pinch of salt into the ganache, mixing until fully combined.
9. Top the Mousse: Once the mousse has set, carefully spoon the chocolate ganache over the top of each serving.
10. Chill Again: Return the mousse to the refrigerator for an additional 30 minutes to let the ganache firm up.

By following these steps, you will create a delightful Pistachio Mousse with Chocolate Ganache that is sure to impress!