Chop pistachios until coarsely chopped. Set aside.
In a mixing bowl, cream together the butter, confectioners' sugar, vanilla bean powder, vanilla bean paste, and sea salt until fluffy for about 3 minutes.
Gradually add the sourdough discard, mixing until fully incorporated.
Combine the soft wheat flour and all-purpose flour in a separate bowl.
In 3 batches, add the flour mixture into the wet ingredients. Mix until just combined at a low speed.
Fold in the chopped pistachios until evenly distributed throughout the dough.
Lay a piece of plastic wrap or parchment on the counter and place the dough on it. Shape it into a log about 1 1/2 inches in diameter. Wrap the log and refrigerate for at least 1 hour or until firm.
Preheat your convection oven to 325°F (165°C). For a conventional oven, 350°F (175°C) Line a baking sheet with parchment paper.
Brush the outside of the dough log with the beaten egg yolk. Roll the log in the turbinado sugar until coated.
Cut the log into slices about 1/2 inch thick and place them on the prepared baking sheet.
Bake for 12-15 minutes for convection or around 20 minutes for conventional or until the edges are lightly golden. If using multiple baking sheets, rotate in the oven from top to bottom and front to back halfway through baking.
Remove from the oven and let the cookies cool on a wire rack for at least 30 minutes before serving.