ACTIVE TIME: 1 hr, 45 mins
TOTAL TIME: 3 hrs, 15 mins
YIELD: Makes 68 1- by 3-in. bars or 48 2-in. squares
1 1/4 pounds (5 sticks) salted butter, at room temperature
1 1/4 cups granulated sugar
1 1/4 cups white rice flour, like Bob's Red Mill
4 cups all-purpose flour
2 tablespoons coarse white sparkling sugar
Preheat oven to 275°. In a stand mixer, beat butter and granulated sugar on medium speed with paddle attachment until well blended and fluffy. Add rice flour and beat 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-in. 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 oven to 250°, drape shortbread with parchment paper, and continue to bake until a finger pressed lightly into center of dough leaves no imprint, 20 to 25 minutes more.
Set shortbread on a cooling rack and sprinkle with sparkling sugar. While hot, cut into 1- by 3-in. rectangles or 2-in. squares (use a wooden ruler). Let cool completely in pan. After it cools, if you find that you'd like it crunchier, pop pan into a 250° oven, covered with parchment, for about 20 minutes.
Make ahead: Airtight at room temperature, up to 1 week, or frozen, up to 3 months.
Per rectangular cookie: calories 113, caloriesfromfat 54 %, protein 1 g, fat 6.9 g, satfat 4.3 g, carbohydrate 12 g, fiber 0.3 g, sodium 48 mg, cholesterol 18 mg