In a food processor, add the flour, sugar, salt, zest, and thyme and pulse until combined.
While the food processor is running, add the cold cubbed vegan butter one by one until the mixture comes together in a rough ball.
Press the dough into a 9-inch fluted tart pan with a removable bottom until it is even all over.
Bake for 35 to 40 minutes, or until the cookie dough looks firm, dry, and slightly golden brown on the edges.
Remove the shortbread from the oven and straight away, with a sharp knife, cut 16 equal pieces. Leave the shortbread to cool completely in the tart tin before removing the cookies by lifting up the removable bottom.
Icing
While the cookies are cooling down, prepare the icing by adding slowly the grapefruit juice to the sifted icing sugar while whisking vigorously. Adjust the quantity of grapefruit juice to your desired consistency. If using, add food coloring to your desired shade. Place the icing in a large bowl where you will be able to maneuver and dip the shortbread cookies in.
Holding the cookies by its side, slowly dip the top part of the cookie until completely covered. Place on a drying rack and continue dipping until all the cookies have had their first coat of icing.
Starting back with your first dipped cookie, repeat the process so that each cookie receives a total of 2 icing dip. Garnish the top of each cookie with fresh thyme or pearl sugar and leave to dry until firm. Enjoy!