1. Prepare the Crust:

1. Preheat oven to 350°F. Line a baking sheet with parchment paper.
2. In a bowl, combine ground pistachios, sugar, flour, and salt. Set aside.
3. In a separate bowl, whisk egg yolks with vanilla extract until thick and pale. Fold in the pistachio mixture and melted butter.
4. In a clean bowl, beat egg whites to stiff peaks. Gently fold into the pistachio batter.
5. Spread batter onto prepared baking sheet into an even layer. Bake for 12-15 minutes until set and lightly golden. Let cool completely, then cut into 12 small circles (2-inch diameter). Set aside.
6. For the vanilla mousse: In a small bowl, bloom gelatin in 1/4 cup water. In a saucepan, heat heavy cream and milk until just simmering. Remove from heat and stir in bloomed gelatin until dissolved. Add white chocolate chips and stir until smooth. Cool to room temperature.
7. Whip the remaining 1/2 cup heavy cream to soft peaks. Fold into cooled white chocolate mixture. Fold in green food coloring until you achieve emerald green.
8. Line a muffin tin with plastic wrap. Place one pistachio cookie circle at the bottom of each cup. Fill with mousse mixture and top with another cookie circle. Press gently. Refrigerate for at least 2 hours.
9. For mirror glaze: In a saucepan, combine sugar, condensed milk, and water. Heat to 225°F (soft ball stage). Remove from heat and stir in bloomed gelatin (gelatin + 1/4 cup water) until dissolved. Add white chocolate chips and stir until smooth. Tint with green food coloring to desired emerald shade. Let cool to 90°F.
10. Remove pillows from muffin tin and place on a wire rack over a baking sheet. Pour warmed mirror glaze over each pillow to coat completely. Let set for 5 minutes. Transfer to serving plates and refrigerate until glaze is firm. Serve chilled.