Preheat your oven to 180° and grease 4 cupcake wholes (in a cupcake tray) with some vegetable oil then dust them with a bit of flour to ensure that the tartlets won't stick.
In a bowl, mix the flour with sugar, vanilla and salt. Add the oil and mix again until a dough forms. Add a bit of water if dough is crumbly and won't come together.
Divide evenly between the cupcake wholes and shape them like tartlets.
Bake in preheated oven for 20-25 mins until a bit dark on the edges.
Remove from oven and let them cool before trying to remove them from the tray. Meanwhile, make the plum filling.
In a saucepan on medium heat, mix all filling ingredients. Keep stirring until plums are a bit darker and release their juice (about 10 mins).
Leave it to cool before filling the mini tarts.
When the tartlets are completely cool, flip the tray and tap it on the kitchen counter or table to release them.
Fill with the plum filling and enjoy!