Note: you can refrigerate the dough before baking if your cookies are spreading too much. We didn't have that issue.
Preheat oven to 375 degrees F
In a large bowl, beat together the butter and sugar until light and airy, about two minutes.
Add the vanilla extract and egg and beat until incorporated.
In a small bowl, stir together the flour, baking powder and salt.
Add the flour mixture to the butter mixture and beat until just combined.
Add the shelled pistachios and white chocolate chips and stir together until combined.
Use a cookie scoop to portion the cookies onto lined cookie trays.
Bake for 12 minutes, or until cookies are just starting to turn golden brown at the edges.
Allow cookies to cool on the cookie sheets for two minutes before removing to a cooling rack.