Special equipment: An 11-inch fluted tart pan with a removable bottom
Preheat the oven to 350 degrees F. Butter an 11-inch fluted tart pan with a removable bottom. In a large bowl, with an electric mixer on medium, beat the butter and sugar until combined and fluffy, about 2 minutes. Add the egg yolks and orange zest and beat until combined. Beat in the flour, salt and baking powder. Add the pistachios and beat just until evenly distributed.
Tip the dough into the prepared pan and press it into an even layer. Freeze until firm, about 15 minutes.
Sprinkle the top of the dough with sanding sugar and bake on a rimmed baking sheet until the shortbread is golden brown and set, 20 to 25 minutes. Transfer the baking sheet holding the tart pan to a rack to cool for a few minutes. While the dough is still warm, remove the pan edges and cut the cookie into wedges. Let cool completely.
Carefully separate the wedges and set them on a work surface. Drizzle with melted chocolate. Let stand at room temperature until the chocolate has set, about 1 hour.