Preheat oven to 350 degrees F (175 degrees C). Grease one 10 inch springform pan.
Sift the flour, xanthan gum, baking soda, and salt together. Set aside.
Separate the eggs. In a large bowl beat the egg whites until soft peaks form. Gradually add 1/2 cup of the sugar, beating until stiff glossy peaks of Merengue form.
In a separate bowl, cream butter, 1 1/2 cups sugar, egg yolks, lemon zest, and lemon juice together until fluffy. Add flour mixture alternately with the yogurt to the egg yolk mixture. Gently fold in the egg whites and pour the batter into the prepared pan.
Bake at 350 degrees F (175 degrees C) for 50 to 60 minutes. Let cake cool in pan for 10 minutes, then turn out onto a rack to finish cooling. Serves 12.
For a fancier cake- slice cake in half. Frost bottom half with lemon curd. Place top of cake back into place. Frost with whipped cream. (Be sure to refrigerate)