While the chocolate mixture cools, blend one cup of the pistachios in a powerful blender (Blendtec or Vitamix) until you have pistachio butter. Try to stop blending just when it turns into a paste and before too much oil is released.
If you do not have a powerful blender , you can use a food processor and process until you get a fine powder. The resulting brownie will not be fudgy, but more cakey, with pistachio morsels. Fret not, it will still be delicious!