I can't remember which US website I got this recipe. Has some handy tips at the end for GF baking.

1 1/3 cups white rice flour

1/2 cup sweet rice flour (I just used the same rice flour)

1/2 cup cornflour

1 teaspoon salt

1/4 teaspoon baking powder

1/4 teaspoon xanthan gum (this prevents the cookies from spreading)

7 ounces (14 tablespoons) butter, softened

3/4 cup granulated sugar

1 teaspoon vanilla extract

Preheat oven to 150°C. Line 2 baking sheets with parchment paper. In small bowl, whisk together rice flour, cornflour, salt, baking powder, and xanthan gum. In another bowl cream butter, sugar, and vanilla extract together, on medium-low speed with an electric whisk, until a thick paste forms, about 30 seconds. Add dry ingredients. Turn on mixer and mix until a dough forms, about three minutes.

Lightly white rice flour your countertop. Turn dough out onto counter and shape dough into a disk. Using a rolling pin, roll dough out, about 1/4-inch thick. Cut using a round cutter and place cookies on prepared baking sheet, about two inches apart. (I added half a blanched almond on top.) Bake until cookies are lightly golden brown on the edges and aromatic, about 30 minutes. Allow cookies to cool on the pan for five minutes and then transfer to wire rack to cool.

Tips

1. Don't overmix. Shortbread cookies are dense affairs. Over-mixing the butter causes their texture to become too light and crumbly. When preparing the dough, don't cream the butter and sugar so much that it becomes light and fluffy.

2. Take your time. Since these cookies contain no eggs, there's no moisture to coax the ingredients quickly into a dough. Once the dry ingredients are added to the butter and sugar paste, it takes a little while for a dough to form. At first the dough seems incredibly sandy--you might even worry that it's not coming together—don't fret like I did the first time I made a batch! The dough does come together after two or three minutes of mixing. Don't rush it and don't give up. The wait's worth it.

3. Re-rolling is welcome. With no gluten to make things tough, you can roll and re-roll this dough until every last piece is used. Since these cookies are so rich, I make about two dozen 2 1/2-inch cookies from one batch, gathering and re-rolling the dough as needed to get this many cookies.