Whisk all-purpose flour, cornstarch, and rice flour in a medium bowl to combine. Using a bench scraper or pastry blender (or your fingers if you don’t have either), work butter, sugar, and salt in a large bowl until butter is cut into smallish pieces and sugar is no longer visible. Add dry ingredients to butter mixture and cut into butter until a rough dough starts to form with some loose flour visible. Turn out onto a clean surface and, working with small handfuls at a time, smear dough across surface with the heel of your hand to work butter and flour together. Continue to work until all the dough is blended. Press into a rectangle, being careful not to overwork dough; you want it to just hold together. Wrap tightly in plastic and chill until firm, about 30 minutes.
Preheat oven to 325°. Roll out dough between 2 sheets of parchment paper to about a 10×8″ rectangle ⅓”–½” thick. Turn over a large baking sheet and place dough on top. Remove top sheet of parchment; bake shortbread, rotating baking sheet once, until pale but not golden and nearly firm to the touch, 25–35 minutes. Remove shortbread from oven and cut into 3×1¼” bars, separating them slightly once cut. Bake, rotating baking sheet once, until deep golden brown, nutty-smelling, and very crisp, 25–35 minutes longer. Let cool.
Just before serving, dust shortbread with powdered sugar and top with cacao nibs and preserved lemon peel.
Do Ahead: Shortbreads can be baked 3 days ahead. Store tightly wrapped at room temperature.