In a medium bowl, cream butter and sugar with electric mixer on medium speed.Add egg and vanilla and beat to combine. Add flour and salt and beat on low to combine. Fold in chocolate and pistachios.
Place dough in fridge for an hour to firm up.
Place dough on clean floured surface and roll into long tube. Wrap dough in wax paper and twist ends closed. Place dough into a paper towel roll (this helps the dough keeps its shape) and freeze overnight.
Preheat oven to 350 degrees. Line two baking sheets with parchment paper.
Let dough stand at room temperature for 15 minutes. Roll dough in sanding sugar to coat. Slice log into 1/4 thick rounds. Place about 1 inch apart on cookie sheets.
Place one cookie sheet on upper rack and one on lower rack. Bake for 8 minutes. Swap racks and rotate cookie sheets and bake for another 8 minutes. Continue to bake until cookies are golden brown.
Transfer cookies to wire rack to cool completely.