Cut the vegan butter into cubes. Place the flour and salt into a food processor. Add the butter. Pulse until the mixture resembles coarse breadcrumbs and the butter is evenly distributed.
Turn the food processor onto its lowest speed and add the water one tablespoon at a time until the mixture just comes together. You may not need to add all the water.
Remove the dough, shape into a rough, flat circle, wrap tightly and place in the fridge to chill for at least 30 minutes.
Lightly flour a clean surface and remove the chilled dough from the fridge. Remove and reserve ¼ of the dough. Form the remaining ¾ of the dough into a rough circle again and roll out to around 1.5 inches larger than the diameter of your tart tin.
Carefully lift the pastry with your rolling pin and drape over a 20cm tart tin. Gently push the dough into the corners, and trim off the excess, leaving around a 1cm overhang around the edge of the tart tin. Set aside.
Roll out the remaining ¼ of the pastry on a floured surface until it's around ¼ cm thick. Use a cookie cutter to cut out holly leaves, stars or any other decorative shape. Place on a parchment lined baking sheet and place both the decorations and the tart in the fridge to chill for the remainder of the 20 mins chilling time.
After the tart has chilled, place it on a baking sheet and line the inside gently with baking parchment. Fill the parchment with baking beads or dry, uncooked rice. Place in the preheated oven. Bake for 15 minutes before removing the baking beads by lifting the corners of the parchment and returning the case to the oven for a further 5 minutes. Don't allow the case to brown too much.
Remove the case from the oven and place on a wire rack to cool - do not remove the case from the tin just yet.
Once the tart case is baked, remove the shortcrust decorations from the oven. Prick them gently with a fork, brush them with plant milk and sprinkle with a little granulated sugar (optional). Bake in the oven for 10-15 minutes or until golden brown. Remove from the oven and allow to cool completely.