Add flour, powdered sugar and, salt to the food processor, and pulse until the ingredients are very well combined
Add very cold butter pieces to the processor, and pulse until the flour/butter mixture looks like coarse sand
Add the egg and pure vanilla extract to the processor, and pulse until the mixture just comes together. If the mixture is still grainy after pressing a bit of it between your thumb and index finger, you can add about one tablespoon of cold water to the dough and pulse. The dough should come together
Next, remove the dough from the processor. Shape the dough into a disk and wrap it in plastic wrap.
Then place the wrapped dough in the fridge to chill for at least 30 minutes ( it is important that you let your dough chill so that the gluten in the dough can relax. The relaxed dough will be much easier to roll and, will cut down on shrinking while you're baking the dough)
After the dough has been chilling for at least 30 minutes, bring it out of the fridge and leave it on the countertop for about five to ten minutes before you roll it. The dough may crack if it is too cold while you're rolling it
Roll out the dough so it is slightly wider than the size that you need for your mini tart tin
Pick up the dough carefully and place it in your tart tin.
Press the dough downwards, and firmly against the sides of the mini tart tin
Once the dough is pressed firmly into the tart tin, roll your thumb along the edge of the tart tin to get rid of any excess dough
Chill the dough in the mini tart tins for at least 30 mins
While the dough is chilling in the fridge, preheat your oven to 400F
After the dough in your tart tins has been chilling for at least 30 minutes, prick holes in the dough using a fork. This is also known as docking the dough. Pricking holes in the dough will prevent the dough from puffing up while it bakes
(Optional) If you have extra mini tart tins, you can press them firmly inside the mini tart tins that have dough. So, you would be sandwiching the dough between two mini tart tins before baking. Doing this will also prevent the dough from puffing up, and will help define the tart shape.
Place the mini tarts tins on a baking sheet, and then place them in the preheated oven for 12 minutes
After 12 minutes, bring the tart shells out of oven and check if they are fully cooked. If the tarts are still pale, put them back in the oven and watch them carefully. They are fully cooked when they start to turn light golden color
Let the tart shells cool completely before using them. You can store them in an airtight container in the fridge for up to a week, or in the freezer for up to 3 months