Cream together the margarine and sugar until light and fluffy. Beat in the egg. Add the vanilla, lemon zest, and lemon juice. Mix to combine.
In a separate small bowl, combine the flours, starches, xanthan gum, baking powder and baking soda.
Add the dry ingredients to the wet ones, beating with the mixer on low until all the dry ingredients are moistened.
Cover and chill the dough 1 hour.
If you are making lemon curd from scratch, you can do that while the dough is chilling. After the curd has thickened, remove from heat and pour it into a stainless steel pan or bowl and set it on a wire rack to cool, stirring occasionally. The curd should not be hot when you put it in the cookies, but it’s okay if it’s still warm.
Preheat the oven to 350°. Use non stick cookie sheets, or sheets lined with parchment paper or a silicone mat.
Roll the dough into small ball and place them about 2 inches apart on the cookie sheets. Press your thumb into the center of each ball making a deep impression.
Place some lemon curd in a zip top plastic bag that has a small bit of the corner cut off. Carefully squeeze the bag to fill the center of each cookie with lemon curd.
Bake for 12 minutes or until the edges of the cookies begin to brown slightly. Cool on the cookie sheet for 5 minutes, then transfer to a wire rack.
Recipe by The Gluten-Free Homemaker at http://glutenfreehomemaker.com/lemon-thumbprint-cookies/