Beat sugar, salt, vegan butter and shortening on medium speed until light and fluffy, about 1-2 minutes.
Sift in the flour gradually into the butter-shortening mixture, mixing with a spatula. Stir together with the spatula until the dough comes together. If needed, sprinkle a few drops of ice-cold water so the dough holds together.
Grease an 8 X 8-inch baking dish.
Put the shortbread dough in it and press with your fingers into a smooth, even layer.
Using a fork, pierce holes into the shortbread dough in a decorative pattern.
Bake in a 300-degree oven for 45-50 minutes until the shortbread is pale golden in color and slightly dark at the edges.
Remove to a rack and let cool until it's slightly warm. With a sharp knife, cut almost all the way through to form squares. At this point you can sprinkle the shortbread with 1-2 tsp sugar.
When completely cooled, break into bars and devour!