Cut the cold butter and reserve at room temperature to temper slightly. Line a 13x9-inch baking sheet or baking dish with parchment paper.
Mix the confectioners’ sugar, salt, and flour in a mixer fitted with a paddle attachment. Mix for a few seconds to combine the dry ingredients.
Add the cubed butter and mix on low speed until a smooth dough is formed and butter is fully incorporated. At first, the dough will look extremely flaky and dry; let it keep mixing and it will eventually come together into a dough.
Dump the dough into the baking sheet ordish and spread it evenly to the corners. Cover with plastic wrap and chill overnight.
The next day, heat the oven to 300°F. Dock the dough every inch or so with a fork. Bake until the shortbread is golden brown, 60-75 minutes.
Remove the pan from the oven and let cool for 5 minutes. Turn the shortbread out onto a cutting board, and slice into 4-inch x ¾-inch slices. Sprinkle with raw sugar and transfer to a baking rack to cool completely. Store in airtight containers.