Yield: Makes 68 1- by 3-inch bars or 48 2-inch squares
- 1¼ pounds (5 sticks) salted butter, at room temperature
- 1¼ cups granulated sugar
- 1¼ cups white rice flour
- 4 cups al-purpose flour
- 2 tablespoons coarse white sparkling sugar
Preheat oven to 275º. In a stand mixer, beat butter and granulated sugar on medium speed with paddle attachment until well blended and fluffy. Add rice flour and bead on low speed until incorporated, scraping bowl as needed. Gradually add all-purpose flour, beat on low until blended, scraping bowl occasionally
Line a 12- by 17-inch rimmed baking sheet with parchment paper. Press dough into pan until very even and smooth. Bake until shortbread is golden in center, 60 to 65 minutes. Reduce heat to 250º, drape shortbread with parchment paper, and continue to bake until a finger pressed lightly into center of dough leaves no imprint, 20-25 minutes more.
Set shortbread on a cooling rack and sprinkle with sparkling sugar. While hot, cut into 1- by 3-inch rectangles or 2-inch squares. Let cool completely in pan.