Preheat oven to 350 degrees F. Line two baking sheet trays with parchment paper.

Make pistachio butter. Add pistachios to a food processor or blender and process until nuts break down to a fine powder. Continue to process for 5 minutes, scraping down the sides of the bowl occasionally. Add a teaspoon of oil and ¼ teaspoon salt. Continue to process until completely smooth, another 5-10 minutes. Set aside ¾ cup pistachio butter, reserving the remaining for another use.

Add butter, brown sugar, and granulated sugar in the bowl of a stand mixer. Beat until creamy, about 3-4 minutes. Alternatively, use a hand mixer.

Add pistachio butter, mixing until well combined. Add egg and vanilla, mixing until well combined.

Combine flour, baking powder, and salt in a medium bowl. Add dry ingredients to cookie dough, mixing just until combined. Add dark chocolate chunks, folding in the chocolate until well combined.

Scoop about 2 tablespoons of cookie dough per cookie and space them at least 2 inches apart on the prepared baking sheet. Use the bottom of a spatula to flatten the cookies. Bake for 8-10 minutes or until light golden brown. Remove from the oven and let cool completely on the baking sheet.