Line the bottom and sides of a 14" rectangular cake tin (with removable base) with some parchment paper. You can also use a 9 or 9.5" round tart tin.
Add the cookies and sea salt to your food processor and blitz for about 30 seconds until evenly finely ground. Add in the vegan butter and blend again until evenly combined and the mixture sticks together between your fingers. Press the cookie mixture into the base and sides of the tart tin, and smooth it out with the back of a spoon, making sure it is compact. Set it aside in the freezer to set for 10 minutes while you prepare the filling.
To a saucepan, add the non-dairy milk, cornstarch, and agar-agar powder and whisk well to remove any lumps. Add the brown sugar and whisk again until combined.
Add the mango pulp and passion fruit juice and bring to a simmer on medium-high heat. Cook for 5 minutes, stirring frequently with a rubber spatula or whisk to prevent the mixture from burning to the pan. Pass the mango curd through a fine-mesh sieve to remove any lumps.
Add the mango curd on top of the cookie crust and smooth out the top of it using the back of a spoon. Place the tart in the fridge to chill for 2-3 hours, or until set.