In a mixing bowl, combine the flour, baking powder, and salt. Whisk to combine. Set aside.
In the bowl of a stand mixer, cream together the butter and sugar. Blend in the milk and vanilla extract. Slowly add the flour mixture. Mix until combined. Fold in chopped chocolate and ½ cup chopped pistachios.
Form the dough into two logs, each approx. 2 inches in diameter. Roll the logs in the remaining ½ cup chopped pistachios. Wrap in waxed paper and chill for 8 hours or up to 3-4 days. (Alternatively, the dough can be frozen for up to 3 months.)
Preheat oven to 375 degrees. Slice the cookies ¼ inch thick. Place cookie dough on parchment-lined baking sheets. Bake for 9-10 minutes until just set. Cool on the sheet pan for 2-3 minutes. Transfer the cookies to a rack to cool completely. Store in an airtight container.