How to Make It
Preheat oven to 350°.
Weigh or lightly spoon brown rice flour into a dry measuring cup; level with a knife. Combine flours, potato starch, cornstarch, xanthan gum, and baking powder in a medium bowl; stir with a whisk.
Place granulated sugar and egg yolks in a large bowl; beat with a mixer at medium speed 3 minutes or until mixture becomes pale yellow. Add vanilla; beat until blended.
Place cream of tartar and egg whites in a medium bowl; beat with a mixer at medium speed until soft peaks form using clean, dry beaters. Slowly add powdered sugar; beat until stiff peaks form. Fold egg white mixture and flour mixture into egg yolk mixture, stirring just until blended.
Spoon batter into a pastry bag fitted with a 1/2-inch round tip; pipe 24 (2 1/2 x 1-inch) cookies 1 inch apart on baking sheets lined with parchment paper. Bake at 350° for 12 minutes or until edges of cookies are golden brown. Cool 2 minutes on pan. Remove from pans, and cool completely on wire racks.