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

For the Pistachio Mousse:

1. Prepare Pistachios: In a food processor, pulse the shelled pistachios and 2 tablespoons of sugar until finely ground.
2. Whip Cream: In a mixing bowl, whisk 1 cup of heavy cream until soft peaks form. Set aside.
3. Whip Egg Whites: In another bowl, beat the egg whites until they are frothy. Gradually add 1/4 cup sugar and continue beating until stiff peaks form.
4. Combine Mixtures: Fold the ground pistachio mixture into the whipped cream, then gently fold in the beaten egg whites. Add vanilla extract and a pinch of salt, mixing until well combined.
5. Chill: Spoon the mousse into serving cups and refrigerate for at least 2 hours to set.

For the Chocolate Ganache:

1. Heat Cream: In a small saucepan, heat 1/2 cup of heavy cream over medium heat until just boiling.
2. Add Chocolate: Remove from heat and stir in the semi-sweet chocolate chips. Let it sit for 2 minutes, then stir until smooth and glossy.
3. Cool Ganache: Allow the ganache to cool slightly before using it to top the mousse.

Assembling the Dessert:

1. Top Mousse with Ganache: Once the mousse has set, pour the chocolate ganache over the top of each cup.
2. Chill Again: Return to the refrigerator for an additional 15-30 minutes to allow the ganache to set slightly.

This simple step-by-step process will guide you in creating this incredible dessert with ease!