Preheat your oven to 325°F. Line rimmed baking sheets with parchment paper and set them aside.

In a large bowl, place the flour, xanthan gum, cornstarch, salt, baking powder, sugar, and lemon zest, and whisk to combine well.

Add the butter, egg, vanilla, and lemon juice, and mix until the cookie dough comes together. It should be smooth and thick. Add more flour by the half-teaspoonful if necessary to help the dough have the proper feel.

Place the dough on a flat surface and roll with a rolling pin into a rectangle a bit less than 1/3-inch thick.

Using a 2 1/2-inch round cookie cutter, cut out rounds of dough and place them about 1-inch apart on the prepared baking sheets. Gather and reroll any scraps, then cut out more shapes until you only have scraps too small to reroll.

Place the baking sheet in the center of the preheated oven and bake for 10 to 12 minutes, or until the bottoms and edges of the cookies are firm and pale golden.

Allow to cool on the baking sheets for about 5 minutes, then transfer to a wire rack to cool completely.

When the cookies are nearly cool, make the icing. In a small bowl, place the confectioners’ sugar and 2 teaspoons of lemon juice or water, and mix well.

Add more liquid by the half-teaspoon until you have a smooth but very thickly pourable glaze. Add the optional food coloring and mix until the color is uniform.

Using a wide spoon or a small offset spatula, spread the glaze in an even layer on the top of each cookie.

Set the cookies on a flat surface and allow the icing to set. To fully set, it may take up to 12 hours. It won’t set as hard as royal icing, though.