For the cookie dough, melt the butter and cool until tepid or barely warm to the touch, for about 40 minutes.
Grind the pistachios and sugar in a food processor, pulsing on/off until the nuts are coarsely chopped, or chop the nuts by hand and mix them with the sugar. Blend the flour and the pistachio/sugar mixture using an electric mixer. Add the tepid butter, orange juice and Grand Marnier and mix just until evenly moistened. The dough will be wet and sticky. Spread the dough out into a 1-inch wide log on a sheet of plastic wrap. Using the wrap as an aid, roll the dough up in plastic. Refrigerate or freeze until firm, for about 6 hours. (This can be prepared ahead and refrigerated 1 week or wrapped in freezer paper and frozen several months. Defrost in the refrigerator before continuing with the recipe.)
To bake, preheat the oven to 325 degrees. Line 2 large baking sheets with parchment paper. Slice the dough into 1/8-inch-thick coins. Arrange as many as will fit on the prepared sheets, leaving 2 inches between the cookies. Dip a fork in water, shake dry and use it to flatten the cookies. Bake the cookies until well browned, for about 12 minutes, exchanging the position of the baking sheets in the oven halfway. Let the cookies cool on parchment until firm enough to dislodge with a spatula, for about 5 minutes. Gently transfer to racks in a single layer. Arrange and bake the remaining coins in the same manner. Cool the cookies completely on racks. Store in an airtight container at room temperature.