In a medium bowl, whisk the flour, baking powder, baking soda, cream of tartar, and salt together until combined. Set aside.
In a large bowl using stand mixer fitted with a paddle attachment or a hand mixer, beat the butter, granulated sugar, and brown sugar together on high speed until completely smooth and creamy, 2-3 minutes. Do not skip this step. Beat in the eggs and vanilla extract on high speed, scraping down the sides of the bowl as needed.
Slowly mix the dry ingredients into the wet ingredients until combined. Slowly beat in the white chocolate chips and pistachios. Cover dough plastic wrap and chill in the refrigerator for 1 hour or up to 3 days.
Once chilled, remove cookie dough from the refrigerator. If the cookie dough chilled longer than 3-4 hours, let it sit at room temperature for about 20 minutes to soften a bit so it's easier to roll.
Preheat oven to 350°F and line a large baking sheet with parchment paper or a silicone baking mats and set aside.
Scoop each cookie dough into 1-2 tablespoon size balls and arrange on the prepared baking sheet about 3 inches apart. Bake for 11-13 minutes until just set and golden brown on the edges.
Allow cookies to cool for 5 minutes on the baking pan then transfer to a wire rack to cool completely.
While they're cooling, Place another large piece of parchment paper out and set aside. Melt the white chocolate baking bar on the stove over medium-low heat or in the microwave in 30-second increments until smooth. Transfer to a taller bowl or wide cup and dip half of each cookie into the white chocolate then place on the prepared parchment paper. Sprinkle with crushed pistachios and place in the refrigerator or freezer to set.
Baked cookies or cookie dough stay good covered in an air tight container in the freezer for up to 3 months.