In a bowl, mix melted butter, sugar, vanilla extract, and salt together in a medium bowl. Add the flour and stir to completely combine. into prepared pan, making sure the layer of crust is nice and even. Bake for 20-22 minutes or until the edges are lightly browned.
To make the lemon filling
In a medium size sauce pan add dairy-free milk, lemon juice, lemon zest, cornstarch (dissolved in a bit of water), agar powderm vanilla extract and a pinch of turmeric. Whisk everything together and bring it to boil over medium heat. Stir constantly and cook for about 3 minutes or until it starts to thicken. Pour the filling over the cooled shortbread crust.
Allow to cool to room temperature, then place it in the fridge for about 2 hours or overnight. Slice into bars and enjoy!