Preheat oven to 350℉ (180℃)
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.
1 ¾ Cup All-purpose flour, ½ teaspoon Baking powder, ¼ teaspoon Baking soda, ½ teaspoon Salt
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)
6 Tablespoons Unsalted butter, 1 Cup Granulated sugar
Add the egg and vanilla extract. Mix on low speed until incorporated.
1 Large Egg, room temperature, 1 teaspoon Pure vanilla extract
Add the sour cream. Mix on low speed until incorporated.
½ Cup Full-fat sour cream
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.
1 Cup Pistachios, shelled, ⅔ Cup White chocolate chips
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.