Heat a conventional oven to 350°F / 180°C and line two baking trays. Stir together flour, sugar, xanthan and salt. Work in the softened butter until sandy, then add the egg and vanilla and mix into a smooth, nonsticky dough.

Divide the dough into three equal portions. Knead ginger into one, cinnamon into the second and cocoa into the third, so each portion has an even color.

Arrange small pieces of all three doughs in a patchwork rectangle. Roll lightly to join, fold into thirds and repeat only enough to create visible marbling. Stop before the colors blend together.

Roll on a lightly floured surface to 1/4–1/3 inch / 6–8 mm thick. Cut with approximately 3 1/2-inch leaf cutters. Arrange an inch apart. Gather scraps without kneading heavily, and reroll. Lightly score veins with a blunt knife or toothpick.

Bake 10–14 minutes: the shorter end gives a softer center; the longer end makes a crisper cookie. Look for lightly golden edges. Cool on the tray for 10 minutes, then finish cooling on a rack.