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. Carefully cut hot shortbread with 2 1/2- to 3-in. chunky letter-shaped cookie cutters* to spell holiday messages such as fa la la, joy, or the initials of recipients. Use a small flexible spatula to help release cutters from dough. With a small knife, cut scraps around letters into smaller pieces. 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.
Carefully remove scraps, then letters, using a small offset spatula, and cut out scraps from within intricate letters like H with knife. Serve scraps in a bowl (they're calorie-free, right?).
Make ahead: Airtight at room temperature, up to 1 week, or frozen, up to 3 months.
*We used 3-in. Ann Clark alphabet cookie cutters (surlatable.com).
Note: Nutritional analysis is per cookie.