Whip the butter and sugar until the mixture is foamy, add the egg and yolk and work the mixture until it is smooth. Add the grated lemon rind.
Mix the flour with the xanthan, add to mixture and stir briefly. Let the dough sit in the refrigerator for an hour. The dough should be soft.
Now roll into balls the size of a walnut and place them on a baking sheet lined with oven parchment, at some distance from each other, because the biscuits will tend to widen.
Preheat the oven, then bake the biscuits at a temperature of 170° C (with both lower and upper elements) for 12-14 minutes, until the lower edge takes on a lovely amber colour. Mix powdered sugar with the lemon juice to create a glaze that is not too dense.
Sprinkle the still warm biscuits with the glaze and let them cool on the baking sheet. Store them in a tin.
Tip: Instead of lemons you can also use organic oranges.