Preheat the oven to 350ºF or 180ºC and line a 9×13 baking dish (about 23x33 cm) with parchment paper. Set aside.
Add all the shortbread crust ingredients to a large mixing bowl and mix until well combined.
Transfer the dough to the baking dish and press firmly, making sure the layer of crust is nice and even.
Bake for 20 to 25 minutes or until the edges are lightly browned. Remove from the oven and set aside.
Add all the lemon filling ingredients to a saucepan and whisk until well combined.
Bring to a boil, then simmer, whisking constantly until the mixture thickens.
Spread the filling over the crust evenly, using a spatula if needed to smooth.
Bake for 15 to 20 minutes.
Cool completely at room temperature and stick in the fridge for 2 more hours until pretty chilled (ideally overnight).
Lift the vegan lemon bars out of the baking dish with the parchment paper, dust with powdered sugar (optional), and cut into squares.
Serve your vegan lemon bars immediately.
Keep the leftovers covered in the fridge for up to 1 week or in the freezer for 3-4 months.