Sprinkle some flour on a clean working area, unwrap your pie dough, and place it flat on the floured surface.
Add the milk to a small stockpot and warm until tiny bubbles form- do not bring it to a boil.
Using a 3-4" diameter glass or (cookie cutter), cut out pie dough circles; repeat until they are all cut out. You will need to roll out "scraps" to make the rest.
Add the dough circles to muffins tins and set aside.
Separate one egg and save the egg white inside a bowl. Brush the egg white onto the edges of the dough.
Crack the eggs into a large glass bowl, add the sugar, and vanilla extract.
Using a hand blender, mix eggs until fluffy, slowly add in the milk and combine.
Carefully fill each dough cup with mixture. You want to make sure you do not overfill the cups or the custard will caramelize, turn dark, and make them stick. TIP: Use an ice cream scoop on this step to make it easier.
Once the dough cups are all filled (you will have extra batter for the remaining tarts), sprinkle a pinch of nutmeg to the top of each.
Place them in the oven on the center rack. Bake at 400°F for 15 minutes.
Reduce heat to 350 degrees and bake for an additional 10 minutes.
Check doneness and remove if done. Depending on dough thickness, you might need to bake them for a few more minutes. Cook them for additional one-minute increments until fully set and golden brown.
Once done, fill the remaining mixture (if applicable) into dough cups and repeat the baking process.
Allow custard tarts to cool for at least 5 minutes inside the muffin tins before removing them. A plastic knife helps to pry them loose if needed.