Preheat oven to 350°F (175°C) and line baking sheets with parchment paper.
In a medium bowl, whisk together flour, baking soda, and salt.
In a large bowl, cream the butter, brown sugar, and granulated sugar until light and fluffy (2–3 minutes).
Beat in the pistachio pudding mix until fully incorporated.
Add eggs one at a time, then mix in vanilla extract.
Gradually add dry ingredients to the wet mixture, stirring until just combined.
Fold in mix-ins of your choice using a spatula.
Scoop dough (about 2 tbsp per cookie) onto baking sheets, spaced 2 inches apart.
Bake for 10–12 minutes, until edges are set and centers are slightly underdone.
Cool on baking sheets for 5 minutes, then transfer to a wire rack to cool completely.