Preheat oven to 425°F/220°C.
Whisk the flour, xanthan, baking powder, salt and sugar together.
With a pastry blender, add the butter into the flour mix until crumbly.
Stir in milk. Before you completely combine the dough together, add the zest and cranberries. Mix until flour is completely combined.
Gently pat dough into a circle (approximately 8–inch/20-cm diameter).
Slice the scones into 6 pieces and place on a greased baking sheet. I didn't catch this step for some reason and I actually just partially cut though the round for 6 scones and ended up cutting them all the way through after baking. It worked fine.
Coat a thin layer of the “egg” wash by whisking the water and cornstarch together, and then brushing onto the dough with a brush.
Bake 15 to 20 minutes or until light brown.
Allow cooling on a baking rack for 10 minutes.
Coat the tops of the scones with the sugar glaze.