Preheat the oven to 350 degrees F (180 degrees C). Line 2 baking sheets with parchment paper.
In a medium saucepan, melt the vegan butter, sugar, and molasses over a low heat. Stir frequently.
Once melted, transfer to a large mixing bowl. Add the flax egg and vanilla extract. Beat with a wooden spoon until well combined.
Sieve in the gluten-free flour, and add cinnamon, ground ginger, baking soda, and xanthan gum. Mix together first with a wooden spoon, then by hand to form a dough.
Shake the dough into a ball. Chill in the fridge for 30 minutes.
Split the dough in half, and roll one section with a rolling pin. Cut out shapes using cookie cutters and transfer them to the lined baking sheets.
Bake for 8 to 10 minutes — 8 minutes will result in a softer cookie, and 10 minutes will make them more crisp. Let cookies cool completely on a wire rack before decorating them.
To make the vegan royal icing, sieve the powdered sugar into a large mixing bowl. Add the almond milk and whisk until smooth. Transfer icing to a piping bag with a small tip and decorate as desired.