How to Make It
Preheat oven to 375°.
Weigh or lightly spoon flours into dry measuring cups; level with a knife. In a medium bowl, sift together flours, cornstarch, and xanthan gum.
Combine sugar and butter in a large bowl; beat with a mixer at medium speed until light and fluffy. Add vanilla, eggs, and egg white, beating until blended. Add flour mixture, beating at low speed until blended.
Drop dough by tablespoonfuls 2 inches apart onto baking sheets lined with parchment paper. Bake at 375° for 13 to 15 minutes or until cookies are lightly browned on edges. Cool completely on pans.
Place chocolate chips in microwave-safe bowl; microwave at HIGH 1 minute. Remove and stir. If needed, microwave 15 seconds and stir; repeat until chocolate is melted and smooth. Spoon melted chocolate on top of each cookie; place on parchment paper until set.