Preheat your oven to 350°F (175°C).
In a large bowl, mix the flour, sugar and salt.
Add the vegan butter and mix until the mixture resembles wet sand.
Press the mixture evenly into the bottom of a lined 9"x9" baking pan.
Bake for about 20 minutes, or until the edges are golden brown.
Remove from the oven and let it cool slightly.
While the shortbread is baking, prepare the filling. In a saucepan, whisk together the sugar, cornstarch and flour until well combined.
Gradually add the lemon juice, mango puree and non-dairy milk, whisking continuously to prevent lumps.
Place the saucepan over medium heat and cook the mixture, stirring constantly, until it thickens and comes to a gentle boil (about 5–10 minutes). Once thick and glossy, remove from heat.
Pour the curd over the pre-baked crust, spreading it evenly.
Return the pan to the oven and bake for an additional 25–30 minutes, or until the filling is set with a slight wobble in the centre.
Allow the bars to cool completely in the pan, then refrigerate for at least 4 hours or overnight to firm up.
Once set, cut into squares and serve.