Beat the softened butter with the brown and granulated sugars until the mixture is pale and fluffy. This process usually takes about three minutes and builds the necessary air pockets for a soft cookie rise.
Pour the dry pistachio pudding mix directly into the butter base and blend until the dough turns a uniform bright green. The cornstarch in the mix is the key to keeping these cookies incredibly chewy for days.
Add the room-temperature eggs one at a time, followed by the vanilla extract, beating well after each addition. Using eggs that aren’t cold prevents the butter from seizing and ensures a smooth, consistent dough.
Gradually add the flour, baking soda, and salt to the wet ingredients on a low speed. Stop mixing as soon as the white streaks disappear to avoid overworking the gluten, which can make the cookies tough.
Stir in the white chocolate chips and chopped pistachios by hand using a sturdy spatula. Cover the bowl and chill the dough in the refrigerator for at least thirty minutes to prevent the cookies from spreading too thin in the oven.
Portion the dough into golf-ball-sized rounds and place them on a lined baking sheet. Bake at 350°F for ten to twelve minutes until the edges are just set, then let them cool on the pan to finish setting.
