Make the pistachio thumbprint cookies:

1. Add the pistachios to a food processor and pulse until a medium grind forms. Remove about 2/3 of the pistachios and place into a bowl for rolling later.
2. Continue processing the remaining pistachios until very finely ground. Measure out 1/4 cup and set aside. Discard the rest or add it to the medium ground pistachios.
3. In a large mixing bowl, beat together the butter, confectioners’ sugar, and salt with a hand mixer until light and fluffy, about 2-3 minutes.
4. Add the egg, vanilla, and almond/pistachio extract. Beat again until creamy and well combined.
5. Add the flour and the ¼ cup of finely ground pistachios, then beat until the well combined.
6. Use a small cookie scoop to portion the dough. Roll each portion in the frothy egg white, then roll in the medium ground pistachios until fully coated. Arrange on a parchment lined baking sheet as you coat them.
7. Use your thumb, the back of a wooden spoon, or a measuring spoon to gently press a divot (or “thumbprint”) into the center of each cookie.
8. Cover lightly with plastic wrap and freeze for 30 minutes or refrigerate for at least one hour.
9. Preheat the oven to 375°F. Line two baking sheets with parchment paper. Transfer the cookies to the prepared baking sheets and bake for 9-10 minutes, or until set and lightly golden at the edges. Re-press the divots into the centers with a spoon while hot. 
10. Allow the cookies to cool slightly on the pan for 5 minutes, then transfer to a cooling rack and let them cool fully before filling the centers.

Make the pistachio white chocolate ganache filling:

1. Add the white chocolate, heavy cream, and pistachio cream to a microwave safe bowl. Microwave in 30 second intervals, stirring between each, until melted and smooth.

2. Let the ganache cool slightly, then transfer to a piping bag fitted with a medium round tip. Pipe the ganache into each thumbprint. Allow cookies to cool completely until the centers set before serving.