Preheat the oven to 300˚. Lightly butter the bottom and side of a 9-inch fluted tart pan. Combine the all-purpose and whole-wheat flours, confectioners' sugar and salt in a food processor and pulse until combined. Add the butter and vanilla and pulse until the flour is moistened and the dough just begins to clump together.
Press the dough evenly into the prepared pan. Prick the dough all over with a fork, about 30 times.
Bake until the shortbread is lightly browned and set in the center, 1 hour to 1 hour 10 minutes. Transfer to a rack and let cool 10 minutes in the pan, then sprinkle with the chopped chocolate. Let the chocolate soften, about 5 minutes, then spread into a thin layer with an offset spatula, leaving a 1/2-inch border around the edge. Top with nonpareils; let cool completely.
Remove the tart from the pan and transfer to a cutting board. Cut into thin wedges.