Preheat the oven to 350°F (175°C).

Mix in a medium bowl the gluten-free flour, xanthan gum, and salt; set aside.

Cream together the softened butter, caster sugar, egg, sourdough starter discard, and vanilla extract in a large bowl.

Combine the flour mixture gradually with the butter mixture, using your hands to form a dough.

Roll out the dough on a floured surface to about ¼ inch thick.

Cut out cookie shapes; make sure the tops and bottoms match perfectly.

Transfer the cutouts to a lined baking sheet, leaving minimal space in between.

Bake for 9-11 minutes until the edges start to turn slightly golden.

Dust the tops with powdered sugar and spread jam on the bottoms of half the cookies before sandwiching them together.

Rest the assembled cookies for 12 hours before serving.