Preheat oven to 350°
Prepare 2 baking sheets by lining with parchment paper or a silcone baking mat.
Whisk together flour, baking powder, baking soda and salt in a medium bowl.
Using a stand-mixer with the paddle attachment, begin creaming the butter and granulated sugar until light and fluffy (approximately 2 - 4 minutes on medium speed). Scrape the bowl as needed. (note: a hand-mixer may also be used)
Add the egg and vanilla extract. Mix on low speed until incorporated.
Add the sour cream. Mix on low speed until incorporated.
Add the flour mixture and mix on low speed until combined.
Add pistachios and white chocolate chips to the mixing bowl. Stir until evenly distributed.
Scoop the dough onto the parchment-lined baking sheets using a small cookie scooper (approximately 1 ½ tablespoon per cookie). Leave approximately 2 inches of space between each cookie.
Bake cookies for 12 - 14 minutes or until the tops are a pale golden color.
Let cookies cool on the baking sheets for 5 to 10 minutes, then transfer to a wire rack to cool completely.