Put the sugar into a sauce pan.
Zest 3 lemons completely into the sugar.
Rub the lemon zest into the sugar with your hands to release the essential oils.
Juice the lemons and mix the juice into the saucepan together with 90 grams of the plant milk.
Bring it to a simmer.
In the meantime prepare a slurry made of the second part of the milk and cornstarch. Make sure there a no clumps in the slurry.
Pour the slurry into the simmering liquid, while constantly whisking.
Cook the custard for around 2 minutes.
Take the custard of the heat and stir in the vegan butter.
Fill the custard into the tart shells and immediately cover with cling film. You can use a piping bag for easier filling of the tartlets.
Let them cool at room temperature, then in the fridge.