Make the crust: in a small bowl, beat the vegan butter and icing sugar together into a fluffy cream, then add in the flour. Mix it together into a ball of dough, adding 1 tbsp of water at a time only if needed. Wrap the dough up in cling wrap and refrigerate it for at least 2 hours or overnight (or if you’re tight on time, you can put it in the freezer for 20 minutes).
After it’s chilled, remove from the fridge and press it evenly into the bottom and up the sides of a 9″ round tin or six 4" tart tins. Poke gently across the bottom with a fork. Bake at 350F for 20-30 minutes until nicely browned (or blind bake 20 minutes then bake 10-15 minutes), then set aside to cool.
Make the mousse: in a medium pot over medium heat, mix the coconut milk and chocolate together. Stir frequently for 10 minutes until the mixture is completely smooth, silky, and slightly thickened.
Remove from heat, stir in vanilla, and let it sit for 5 minutes, then pour into the baked tart shells.
Cover and refrigerate for at least 2 hours, or overnight, if you can stand to wait that long.
Decorate: whip the cream and sugar together until fairly thick, then spoon it into a piping bag and make a pattern on top of your chilled tarts. Enjoy!