A touch of rice flour is the secret to perfectly crisp shortbread. With such a short ingredient list, it's important to use the best-quality salted butter you can find.
Preheat oven to 300 degrees. Coat an 8-inch ceramic shortbread mold with cooking spray; set aside.
In the bowl of an electric mixer fitted with the paddle attachment, beat together butter and sugar until well combined. Add flours and mix until just combined and a shaggy dough forms.
Firmly press dough into mold in an even layer. Bake until edges just begin to turn golden, about 1 hour, 15 minutes. Remove from oven; turn oven off. Let shortbread cool in mold 10 minutes.
Invert a parchment-lined baking sheet onto back of mold. Carefully re-invert, turning mold onto sheet; lift off mold. Using a sharp knife, cut shortbread into wedges. Separate wedges, then return sheet to oven until shortbread is dry, at least 1 hour. Let cool completely.