Preheat your oven to 350°F. Line a large rimmed baking sheet with unbleached parchment paper and set it aside.

In a large bowl, place the flour, xanthan gum, cornstarch, baking powder, salt and confectioners’ sugar, and whisk to combine well. Add the lemon zest and mix to combine, breaking up any clumps of zest. Create a well in the center of the dry ingredients and add the butter, egg white and 2 teaspoons of fresh lemon juice, mixing to combine after each addition. The dough will be thick and smooth. Add more lemon juice by the half-teaspoonful and knead it in with clean, dry hands as necessary to bring the dough together.

Scoop the dough by the heaping tablespoon onto the prepared baking sheet, leaving about 1 1/2-inches between pieces. Roll each piece of dough into a round between the palms of your hands, and press down slightly into a thick disk. Press with the moistened tines of a fork into a small bowl of granulated sugar, then press down on the top of each piece of dough until the tines leave an impression about 1/4-inch deep. Place the baking sheet in the center of the preheated oven and bake until the cookies are lightly golden brown on the edges and firm to the touch (about 12 minutes). Remove from the oven and allow to cool completely on the baking sheet.

When the cookies are cool, make the glaze. In a small bowl, place the confectioners’ sugar and 2 teaspoons of lemon juice, and mix well. Add more lemon juice by the half-teaspoon, stirring to combine after each addition until you have a smooth but very thickly pourable glaze. Drizzle the glaze in a zig-zag pattern on top of each cooled cookie. Allow to set at room temperature.