1. Preheat oven to 350°.
2. Weigh or lightly spoon flours into dry measuring cups; level with a knife. Combine flours, potato starch, xanthan gum, baking powder, and salt in a medium bowl, stirring with a whisk.
3. Place sugars and butter in a large bowl; beat with a mixer at medium speed until well blended. Add lemon juice, vanilla, rind, and egg yolks; beat until blended. Add flour mixture, 1/4 cup at a time, beating after each addition.
4. Cover a large baking sheet with parchment paper. Shape dough into 20 balls. Place 2 inches apart on baking sheet. Bake at 350° for 15 to 20 minutes or until lightly browned. Cool 5 minutes on pan. Remove cookies from pan; cool completely on wire racks.